CS计算机代考程序代写 scheme DHCP ITE PC v4.0 Chapter 1

ITE PC v4.0 Chapter 1

Subnetting IP Networks

Introduction to Networks

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
1

Network Segmentation
Reasons for Subnetting
Subnetting is the process of segmenting a network into multiple smaller network spaces called subnetworks or subnets.
Large networks must be segmented into smaller subnetworks, creating smaller groups of devices and services to:
Control traffic by containing broadcast traffic within each subnetwork.
Reduce overall network traffic and improve network performance.
Communication Between Subnets
A router is necessary for devices on different networks and subnets to communicate.
Each router interface must have an IPv4 host address that belongs to the network or subnet that the router interface is connected.
Devices on a network and subnet use the router interface attached to their LAN as their default gateway.

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
2
9.1.1.1 Reasons for Subnetting
9.1.1.2 Communication Between Subnets
Subnetting an IPv4 Network
Network Segmentation
In early network implementations, it was common for organizations to have all computers and other networked devices connected to a single IP network. All devices in the organization were assigned an IP address with a matching network ID. This type of configuration is known as a flat network design. In a small network, with a limited number of devices, a flat network design is not problematic. However, as the network grows, this type of configuration can create major issues.
Consider how on an Ethernet LAN, devices use broadcasts to locate needed services and devices. Recall that a broadcast is sent to all hosts on an IP network. The Dynamic Host Configuration Protocol (DHCP) is an example of a network service that depends on broadcasts. Devices send broadcasts across the network to locate the DHCP server. On a large network, this could create a significant amount of traffic slowing network operations. Additionally, because a broadcast is addressed to all devices, all devices must accept and process the traffic, resulting in increased device processing requirements. If a device must process a significant amount of broadcasts, it could even slow device operations. For reasons such as these, larger networks must be segmented into smaller sub-networks, keeping them localized to smaller groups of devices and services.
The process of segmenting a network, by dividing it into multiple smaller network spaces, is called subnetting. These sub-networks are called subnets. Network administrators can group devices and services into subnets that are determined by geographic location (perhaps the 3rd floor of a building), by organizational unit (perhaps the sales department), by device type (printers, servers, WAN), or any other division that makes sense for the network. Subnetting can reduce overall network traffic and improve network performance.
Note: A subnet is equivalent to a network and these terms can be used interchangeably. Most networks are a subnet of some larger address block.

Objectives
Explain why routing is necessary for hosts on different networks to communicate.
Describe IP as a communication protocol used to identify a single device on a network.
Given a network and a subnet mask, calculate the number of host addresses available.
Calculate the necessary subnet mask in order to accommodate the requirements of a network.
Describe the benefits of variable length subnet masking (VLSM).
Explain how IPv6 address assignments are implemented in a business network.

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
3
Chapter 9
Subnetting IP Networks
Introduction
Designing, implementing and managing an effective IP addressing plan ensures that networks can operate effectively and efficiently. This is especially true as the number of host connections to a network increases. Understanding the hierarchical structure of the IP address and how to modify that hierarchy in order to more efficiently meet routing requirements is an important part of planning an IP addressing scheme.
In the original IPv4 address, there are two levels of hierarchy: a network and a host. These two levels of addressing allow for basic network groupings that facilitate in routing packets to a destination network. A router forwards packets based on the network portion of an IP address; once the network is located, the host portion of the address allows for identification of the destination device.
However, as networks grow, with many organizations adding hundreds, and even thousands of hosts to their network, the two-level hierarchy is insufficient.
Subdividing a network adds a level to the network hierarchy, creating, in essence, three levels: a network, a subnetwork, and a host. Introducing an additional level to the hierarchy creates additional sub-groups within an IP network that facilities faster packet delivery and added filtration, by helping to minimize ‘local’ traffic.
This chapter examines, in detail, the creation and assignment of IP network and subnetwork addresses through the use of the subnet mask.

IP Subnetting is FUNdamental
The Plan

Planning the Network

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
9.1.2.1 The Plan
9.1.2.2 The Plan – Address Assignment
Subnetting an IPv4 Network
Network Segmentation
A router is necessary for devices on different networks to communicate. Devices on a network use the router interface attached to their LAN as their default gateway. Traffic that is destined for a device on a remote network will be processed by the router and forwarded toward the destination. To determine if traffic is local or remote, the router uses the subnet mask.
In a subnetted network space, this works exactly the same way. As shown in the figure, subnetting creates multiple logical networks from a single address block or network address. Each subnet is treated as a separate network space. Devices on the same subnet must use an address, subnet mask, and default gateway that correlates to the subnet that they are a part of.
Traffic cannot be forwarded between subnets without the use of a router. Every interface on the router must have an IPv4 host address that belongs to the network or subnet to which the router interface is connected.
Subnetting an IPv4 Network
IP Subnetting is FUNdamental
As shown in the figure, planning network subnets requires examination of both the needs of an organization’s network usage, and how the subnets will be structured. Doing a network requirement study is the starting point. This means looking at the entire network and determining the main sections of the network and how they will be segmented. The address plan includes deciding the needs for each subnet in terms of size, how many hosts per subnet, how host addresses will be assigned, which hosts will require static IP addresses and which hosts can use DHCP for obtaining their addressing information.
The size of the subnet involves planning the number of hosts that will require IP host addresses in each subnet of the subdivided private network. For example in a campus network design you might consider how many hosts are needed in the Administrative LAN, how many in the Faculty LAN and how many in the Student LAN. In a home network, a consideration might be done by the number of hosts in the Main House LAN and the number of hosts in the Home Office LAN.
As discussed earlier, the private IP address range used on a LAN is the choice of the network administrator and needs careful consideration to be sure that enough host address will be available for the currently known hosts and for future expansion. Remember the private IP address ranges are:
10.0.0.0 with a subnet mask of 255.0.0.0
172.16.0.0 with a subnet mask of 255.240.0.0
192.168.0.0 with a subnet mask of 255.255.0.0
Knowing your IP address requirements will determine the range or ranges of host addresses you implement. Subnetting the selected private IP address space will provide the host addresses to cover your network needs.
Public addresses used to connect to the Internet are typically allocated from a service provider. So while the same principles for subnetting would apply, this is not generally the responsibility of the organization’s network administrator.
9.1.2.2
Subnetting an IPv4 Network
IP Subnetting is FUNdamental
Create standards for IP address assignments within each subnet range. For example:
Printers and servers will be assigned static IP addresses
User will receive IP addresses from DHCP servers using /24 subnets
Routers are assigned the first available host addresses in the range
Two very important factors that will lead to the determination of which private address block is required, are the number of subnets required and the maximum number of hosts needed per subnet. Each of these address blocks will allow you to appropriately allocate hosts based on the given size of a network and its required hosts currently and in the near future. Your IP space requirements will determine the range or ranges of hosts you implement.
In the upcoming examples you will see subnetting based on address blocks that have subnet masks of 255.0.0.0, 255.255.0.0, and 255.255.255.0.

Subnetting an IPv4 Network
Basic Subnetting
Borrowing Bits to Create Subnets
Borrowing 1 bit 21 = 2 subnets

Subnet 1
Network 192.168.1.128-255/25
Mask: 255.255.255.128
Subnet 0
Network 192.168.1.0-127/25
Mask: 255.255.255.128
Borrowing 1 Bit from the host portion creates 2 subnets with the same subnet mask

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
5
9.1.3.1 Basic Subnetting
Subnetting an IPv4 Network
Subnetting an IPv4 Network
Every network address has a valid range of host addresses. All devices attached to the same network will have an IPv4 host address for that network and a common subnet mask or network prefix.
The prefix and the subnet mask are different ways of representing the same thing – the network portion of an address.
IPv4 subnets are created by using one or more of the host bits as network bits. This is done by extending the mask to borrow some of the bits from the host portion of the address to create additional network bits. The more host bits borrowed, the more subnets that can be defined. For each bit borrowed, the number of subnetworks available is doubled. For example, if 1 bit is borrowed, 2 subnets can be created. If 2 bits, 4 subnets are created, if 3 bits are borrowed, 8 subnets are created, and so on. However, with each bit borrowed, fewer host addresses are available per subnet.
Bits can only be borrowed from the host portion of the address. The network portion of the address is allocated by the service provider and cannot be changed.
Note: In the examples in the figures, only the last octet is shown in binary because only bits from the host portion can be borrowed.
As shown in Figure 1, the 192.168.1.0/24 network has 24 bits in the network portion and 8 bits in the host portion, which is indicated with the subnet mask 255.255.255.0 or /24 notation. With no subnetting, this network supports a single LAN interface. If an additional LAN is needed, the network would need to be subnetted.
In Figure 2, 1 bit is borrowed from the most significant bit (leftmost bit) in the host portion, thus extending the network portion to 25 bits. This creates 2 subnets identified by using a 0 in the borrowed bit for the first network and a 1 in the borrowed bit for the second network. The subnet mask for both networks uses a 1 in the borrowed bit position to indicate that this bit is now part of the network portion.
As shown in Figure 3, when we convert the binary octet to decimal we see that the first subnet address is 192.168.1.0 and the second subnet address is 192.168.1.128. Because a bit has been borrowed, the subnet mask for each subnet is 255.255.255.128 or /25.

Subnetting an IPv4 Network
Subnets in Use

Subnet 0
Network 192.168.1.0-127/25
Subnet 1
Network 192.168.1.128-255/25

Subnets in Use

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
6
9.1.3.2 Subnets in Use
Subnetting an IPv4 Network
Subnetting an IPv4 Network
In the previous example, the 192.168.1.0/24 network was subnetted to create two subnets:
192.168.1.0/25
192.168.1.128/25
In Figure 1, notice that router R1 has two LAN segments attached to its GigabitEthernet interfaces. The subnets will be used for the segments attached to these interfaces. To serve as the gateway for devices on the LAN, each of the router interfaces must be assigned an IP address within the range of valid addresses for the assigned subnet. It is common practice to use the first or last available address in a network range for the router interface address.
The first subnet, 192.168.1.0/25, is used for the network attached to GigabitEthernet 0/0 and the second subnet, 192.168.1.128/25, is used for the network attached to GigabitEthernet 0/1. To assign an IP address for each of these interfaces, it is necessary to determine the range of valid IP addresses for each subnet.
The following are guidelines for each of the subnets:
Network address – All 0 bits in the host portion of the address.
First host address – All 0 bits plus a right-most 1 bit in the host portion of the address.
Last host address – All 1 bits plus a right-most 0 bit in the host portion of the address.
Broadcast address – All 1 bits in the host portion of the address.
As shown in Figure 2, the first host address for the 192.168.1.0/25 network is 192.168.1.1, and the last host address is 192.168.1.126. Figure 3 shows that the first host address for the 192.168.1.128/25 network is 192.168.1.129, and the last host address is 192.168.1.254.
To assign the first host address in each subnet to the router interface for that subnet, use the ip address command in interface configuration mode as shown in Figure 4. Notice that each subnet uses the subnet mask of 255.255.255.128 to indicate that the network portion of the address is 25 bits.
A host configuration for the 192.168.1.128/25 network is shown in Figure 5. Notice that the gateway IP address is the address configured on the G0/1 interface of R1, 192.168.1.129, and the subnet mask is 255.255.255.128.

Subnetting an IPv4 Network
Subnetting Formulas

Calculate number of subnets

Calculate number of hosts

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
7
9.1.3.3 Subnetting Formulas
Subnetting an IPv4 Network
Subnetting an IPv4 Network
Calculating Subnets
Use this formula to calculate the number of subnets:
2^n (where n = the number of bits borrowed)
As shown in Figure 1, for the 192.168.1.0/25 example, the calculation looks like this:
2^1 = 2 subnets
Calculating Hosts
Use this formula to calculate the number of hosts per network:
2^n (where n = the number of bits remaining in the host field)
As shown in Figure 2, for the 192.168.1.0/25 example, the calculation looks like this:
2^7 = 128
Because hosts cannot use the network address or broadcast address from a subnet, 2 of these addresses are not valid for host assignment. This means that each of the subnets has 126 (128-2) valid host addresses.

So in this example, borrowing 1 host bit toward the network results in creating 2 subnets, and each subnet can have a total of 126 hosts assigned.

Subnetting an IPv4 Network
Creating 4 Subnets
Borrowing 2 bits to create 4 subnets. 22 = 4 subnets

Creating 4 Subnets

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
8
9.1.3.4 Creating 4 subnets
Subnetting an IPv4 Network
Subnetting an IPv4 Network
Consider an internetwork that requires three subnets.
Using the same 192.168.1.0/24 address block, host bits must be borrowed to create at least 3 subnets. Borrowing a single bit would only provide 2 subnets. To provide more networks, more host bits must be borrowed. Calculate the number of subnets created if 2 bits are borrowed using the formula 2^number of bits borrowed:
2^2 = 4 subnets
Borrowing 2 bits creates 4 subnets, as shown in Figure 1.
Recall that the subnet mask must change to reflect the borrowed bits. In this example, when 2 bits are borrowed, the mask is extended 2 bits into the last octet. In decimal, the mask is represented as 255.255.255.192, because the last octet is 1100 0000 in binary.
Host Calculation
To calculate the number of hosts, examine the last octet. After borrowing 2 bits for the subnet, there are 6 host bits remaining.
Apply the host calculation formula as shown in Figure 2.
2^6 = 64
But remember that all 0 bits in the host portion of the address is the network address, and all 1s in the host portion is a broadcast address. Therefore, there are only 62 host addresses that are actually available for each subnet.
As shown in Figure 3, the first host address for the first subnet is 192.168.1.1 and the last host address is 192.168.1.62. Figure 4 shows the ranges for subnets 0 – 2. Remember that each host must have a valid IP address within the range defined for that network segment. The subnet assigned to the router interface will determine which segment a host belongs to.
In Figure 5 a sample configuration is shown. In this configuration, the first network is assigned to the GigabitEthernet 0/0 interface, the second network is assigned to the GigabitEthernet 0/1 interface, and the third network is assigned to the Serial 0/0/0 network.
Again, using a common addressing plan, the first host address in the subnet is assigned to the router interface. Hosts on each subnet will use the address of the router interface as the default gateway address.
PC1 (192.168.1.2/26) will use 192.168.1.1 (G0/0 interface address of R1) as its default gateway address
PC2 (192.168.1.66/26) will use 192.168.1.65 (G0/1 interface address of R1) as its default gateway address
Note: All devices on the same subnet will have a host IPv4 address from the range of host addresses and will use the same subnet mask.

Subnetting an IPv4 Network
Creating Eight Subnets
Borrowing 3 bits to Create 8 Subnets. 23 = 8 subnets

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
9
9.1.3.5 Creating 8 subnets
Subnetting an IPv4 Network
Subnetting an IPv4 Network
Next, consider an internetwork that requires five subnets as shown in Figure 1.
Using the same 192.168.1.0/24 address block, host bits must be borrowed to create at least 5 subnets. Borrowing 2 bits would only provide 4 subnets as seen in the previous example. To provide more networks, more host bits must be borrowed. Calculate the number of subnets created if 3 bits are borrowed using the formula:
2^3 = 8 subnets
As shown in Figures 2 and 3, borrowing 3 bits creates 8 subnets. When 3 bits are borrowed, the subnet mask is extended 3 bits into the last octet (/27), resulting in a subnet mask of 255.255.255.224. All devices on these subnets will use the subnet mask 255.255.255.224 mask (/27).
Host Calculation
To calculate the number of hosts, examine the last octet. After borrowing 3 bits for the subnet, there are 5 host bits remaining.
Apply the host calculation formula:
2^5 = 32, but subtract 2 for the all 0s in the host portion (network address) and all 1s in the host portion (broadcast address).
The subnets are assigned to the network segments required for the topology as shown in Figure 4.
Again, using a common addressing plan, the first host address in the subnet is assigned to the router interface, as shown in Figure 5. Hosts on each subnet will use the address of the router interface as the default gateway address.
PC1 (192.168.1.2/27) will use 192.168.1.1 address as its default gateway address.
PC2 (192.168.1.34/27) will use 192.168.1.33 address as its default gateway address.
PC3 (192.168.1.98/27) will use 192.168.1.97 address as its default gateway address.
PC4 (192.168.1.130/27) will use 192.168.1.129 address as its default gateway address.

Subnetting an IPv4 Network
Creating Eight Subnets (Cont.)

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
10
9.1.3.5 Creating 8 Subnets (Cont.)

Subnetting an IPv4 Network
Creating Eight Subnets (Cont.)

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
11
9.1.3.5 Creating 8 Subnets (Cont.)

Determining the Subnet Mask
Subnetting Based on Host Requirements
Two considerations when planning subnets:
Number of subnets required
Number of host addresses required

Formula to determine number of usable hosts: 2^n-2
2^n (where n is the number of remaining host bits) is used to calculate the number of hosts.
-2 (The subnetwork ID and broadcast address cannot be used on each subnet.)

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
12
9.1.4.1 Subnetting based on Host Requirements
Subnetting an IPv4 Network
Determining the Subnet Mask
The decision about how many host bits to borrow to create subnets is an important planning decision. There are two considerations when planning subnets: the number of host addresses required for each network and the number of individual subnets needed. The animation shows the subnet possibilities for the 192.168.1.0 network. The selection of a number of bits for the subnet ID affects both the number of possible subnets and the number of host addresses in each subnet.
Notice that there is an inverse relationship between the number of subnets and the number of hosts. The more bits borrowed to create subnets the fewer host bits are available; therefore, fewer hosts per subnet. If more host addresses are needed, more host bits are required, resulting in fewer subnets.
Number of Hosts
When borrowing bits to create multiple subnets, you leave enough host bits for the largest subnet. The number of host addresses required in the largest subnet will determine how many bits must be left in the host portion. The formula 2^n (where n is the number the number of host bits remaining) is used to calculate how many addresses will be available on each subnet. Recall that 2 of the addresses cannot be used, so that the usable number of addresses can be calculated as 2^n-2.

Determining the Subnet Mask
Subnetting Network-Based Requirements
Calculate the number of subnets:
2^n (where n is the number of bits borrowed)
Subnet needed for each department.

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
13
9.1.4.2 Subnetting Network-Based Requirements
Subnetting an IPv4 Network
Determining the Subnet Mask
Sometimes a certain number of subnets is required, with less emphasis on the number of host addresses per subnet. This may be the case if an organization chooses to separate their network traffic based on internal structure or department setup. For example, an organization may choose to put all host devices used by employees in the Engineering department in one network, and all host devices used by management in a separate network. In this case, the number of subnets is most important in determining how many bits to borrow.
Recall the number of subnets created when bits are borrowed can be calculated using the formula 2^n (where n is the number of bits borrowed). There is no need to subtract any of the resulting subnets, as they are all usable.
The key is to balance the number of subnets needed and the number of hosts required for the largest subnet. The more bits borrowed to create additional subnets means fewer hosts available per subnet.

Determining the Subnet Mask
Subnetting To Meet Network Requirements

Balance the required number of subnets and hosts for the largest subnet.
Design the addressing scheme to accommodate the maximum number of hosts for each subnet.
Allow for growth in each subnet.

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
14
9.1.4.3 Subnetting to Meet Network Requirements
Subnetting an IPv4 Network
Determining the Subnet Mask
Every network within an organization is designed to accommodate a finite number of hosts. Basic subnetting requires enough subnets to accommodate the networks while also providing enough host addresses per subnet.
Some networks, such as point-to-point WAN links, require only two hosts. Other networks, such as a user LAN in a large building or department, may need to accommodate hundreds of hosts. Network administrators must devise the internetwork addressing scheme to accommodate the maximum number of hosts for each network. The number of hosts in each division should allow for growth in the number of hosts.
Determine the Total Number of Hosts
First, consider the total number of hosts required by the entire corporate internetwork. A block of addresses large enough to accommodate all devices in all the corporate networks must be used. These devices include end user devices, servers, intermediate devices, and router interfaces.
Consider the example of a corporate internetwork that must accommodate a total of 138 hosts in its five locations (see Figure 1). In this example, the service provider has allocated a network address of 172.16.0.0/22 (10 host bits). As shown in Figure 2, this will provide 1,022 host addresses, which will more than accommodate the addressing needs for this internetwork.

Determining the Subnet Mask
Subnetting To Meet Network Requirements

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
15
9.1.4.4 Subnetting to Meet Network Requirements (Cont.
Subnetting an IPv4 Network
Determining the Subnet Mask
Determine the Number and Size of the Networks
Next, consider the number of subnets required and the number of host addresses needed on each subnet. Based on the network topology consisting of 5 LAN segments and 4 internetwork connections between routers, 9 subnets are required. The largest subnet requires 40 hosts. When designing an addressing scheme, you should anticipate growth in both the number of subnets and the hosts per subnet.
The 172.16.0.0/22 network address has 10 host bits. Because the largest subnet requires 40 hosts, a minimum of 6 host bits are needed to provide addressing for 40 hosts. This is determined by using this formula: 2^6 – 2 = 62 hosts. The first 4 host bits can be used to allocate subnets. Using the formula for determining subnets, this results in 16 subnets: 2^4 = 16. Because the example internetwork requires 9 subnets this will meet the requirement and allow for some additional growth.
When 4 bits are borrowed the new prefix length is /26 with a subnet mask of 255.255.255.192.
As shown in Figure 1, using the /26 prefix length, the 16 subnet addresses can be determined. Only the subnet portion of the address is incremented. The original 22 bits of the network address cannot change and the host portion will contain all 0 bits.
Note: Notice that because the subnet portion is in both the third and fourth octets that one or both of these values will vary in the subnet addresses.
As shown in Figure 2, the original 172.16.0.0/22 network was a single network with 10 host bits providing 1,022 usable addresses to assign to hosts. By borrowing 4 host bits, 16 subnets (0000 through 1111) can be created. Each subnet has 6 host bits or 62 usable host addresses per subnet.
As shown in Figure 3, the subnets can be assigned to the LAN segments and router-to-router connections.

Benefits of Variable Length Subnet Masking
Traditional Subnetting Wastes Addresses
Traditional subnetting – Uses the same number of addresses is allocated for each subnet.
Subnets that require fewer addresses have unused (wasted) addresses; for example, WAN links only need two addresses.

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
16
9.1.4.1 Traditional Subnetting Wastes Addresses
Subnetting an IPv4 Network
Benefits of Variable Length Subnet Masking
Using traditional subnetting, the same number of addresses is allocated for each subnet. If all the subnets have the same requirements for the number of hosts, these fixed size address blocks would be efficient. However, most often that is not the case.
For example, the topology shown in Figure 1 requires seven subnets, one for each of the four LANs and one for each of the three WAN connections between routers. Using traditional subnetting with the given address of 192.168.20.0/24, 3 bits can be borrowed from the host portion in the last octet to meet the subnet requirement of seven subnets. As shown in Figure 2, borrowing 3 bits creates 8 subnets and leaves 5 host bits with 30 usable hosts per subnet. This scheme creates the needed subnets and meets the host requirement of the largest LAN.
Although this traditional subnetting meets the needs of the largest LAN and divides the address space into an adequate number of subnets, it results in significant waste of unused addresses.
For example, only two addresses are needed in each subnet for the three WAN links. Because each subnet has 30 usable addresses, there are 28 unused addresses in each of these subnets. As shown in Figure 3, this results in 84 unused addresses (28×3).
Further, this limits future growth by reducing the total number of subnets available. This inefficient use of addresses is characteristic of traditional subnetting of classful networks.
Applying a traditional subnetting scheme to this scenario is not very efficient and is wasteful. In fact, this example is a good model for showing how subnetting a subnet can be used to maximize address utilization.
Subnetting a subnet, or using Variable Length Subnet Mask (VLSM), was designed to avoid wasting addresses.

Benefits of Variable Length Subnet Masking
Variable Length Subnet Masks (VLSM)
The variable-length subnet mask (VLSM) or subnetting a subnet provides more efficient use of addresses.
VLSM allows a network space to be divided in unequal parts.
Subnet mask varies, depending on how many bits have been borrowed for a particular subnet.
Network is first subnetted, and then the subnets are resubnetted.

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
17
9.1.4.2 Variable Length Subnet Masks (VLSM)
Subnetting an IPv4 Network
Determining the Subnet Mask
Sometimes a certain number of subnets is required, with less emphasis on the number of host addresses per subnet. This may be the case if an organization chooses to separate their network traffic based on internal structure or department setup. For example, an organization may choose to put all host devices used by employees in the Engineering department in one network, and all host devices used by management in a separate network. In this case, the number of subnets is most important in determining how many bits to borrow.
Recall the number of subnets created when bits are borrowed can be calculated using the formula 2^n (where n is the number of bits borrowed). There is no need to subtract any of the resulting subnets, as they are all usable.
The key is to balance the number of subnets needed and the number of hosts required for the largest subnet. The more bits borrowed to create additional subnets means fewer hosts available per subnet.

Benefits of Variable Length Subnet Masking
Basic VLSM

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
18
9.1.5.3 Basic VLSM
Subnetting an IPv4 Network
Benefits of Variable Length Subnet Masking
Using traditional subnetting, the same number of addresses is allocated for each subnet. If all the subnets have the same requirements for the number of hosts, these fixed size address blocks would be efficient. However, most often that is not the case.
For example, the topology shown in Figure 1 requires seven subnets, one for each of the four LANs and one for each of the three WAN connections between routers. Using traditional subnetting with the given address of 192.168.20.0/24, 3 bits can be borrowed from the host portion in the last octet to meet the subnet requirement of seven subnets. As shown in Figure 2, borrowing 3 bits creates 8 subnets and leaves 5 host bits with 30 usable hosts per subnet. This scheme creates the needed subnets and meets the host requirement of the largest LAN.
Although this traditional subnetting meets the needs of the largest LAN and divides the address space into an adequate number of subnets, it results in significant waste of unused addresses.
For example, only two addresses are needed in each subnet for the three WAN links. Because each subnet has 30 usable addresses, there are 28 unused addresses in each of these subnets. As shown in Figure 3, this results in 84 unused addresses (28×3).
Further, this limits future growth by reducing the total number of subnets available. This inefficient use of addresses is characteristic of traditional subnetting of classful networks.
Applying a traditional subnetting scheme to this scenario is not very efficient and is wasteful. In fact, this example is a good model for showing how subnetting a subnet can be used to maximize address utilization.
Subnetting a subnet, or using Variable Length Subnet Mask (VLSM), was designed to avoid wasting addresses.
Subnetting an IPv4 Network
Benefits of Variable Length Subnet Masking
In all of the previous examples of subnetting, notice that the same subnet mask was applied for all the subnets. This means that each subnet has the same number of available host addresses.
As illustrated in Figure 1, traditional subnetting creates subnets of equal size. Each subnet in a traditional scheme uses the same subnet mask. As shown in Figure 2, VLSM allows a network space to be divided in unequal parts. With VLSM the subnet mask will vary depending on how many bits have been borrowed for a particular subnet, thus the “variable” part of the VLSM.
VLSM subnetting is similar to traditional subnetting in that bits are borrowed to create subnets. The formulas to calculate the number of hosts per subnet and the number of subnets created still apply. The difference is that subnetting is not a single pass activity. With VLSM, the network is first subnetted, and then the subnets are subnetted again. This process can be repeated multiple times to create subnets of various sizes.

Benefits of Variable Length Subnet Masking
VLSM in Practice

Using VLSM subnets, the LAN and WAN segments in example below can be addressed with minimum waste.
Each LANs will be assigned a subnet with /27 mask.
Each WAN link will be assigned a subnet with /30 mask.

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
19
9.1.5.4 VLSM in Practice
Subnetting an IPv4 Network
Benefits of Variable Length Subnet Masking
Using the VLSM subnets, the LAN and WAN segments can be addressed without unnecessary waste.
The hosts in each of the LANs will be assigned a valid host address with the range for that subnet and /27 mask. Each of the four routers will have a LAN interface with a /27 subnet and a one or more serial interfaces with a /30 subnet.
Using a common addressing scheme, the first host IPv4 address for each subnet is assigned to the LAN interface of the router. The WAN interfaces of the routers are assigned the IP addresses and mask for the /30 subnets.
Figures 1 – 4 show the interface configuration for each of the routers.
Hosts on each subnet will have a host IPv4 address from the range of host addresses for that subnet and an appropriate mask. Hosts will use the address of the attached router LAN interface as the default gateway address.
Building A Hosts (192.168.20.0/27) will use router 192.168.20.1 address as the default gateway address.
Building B Hosts (192.168.20.32/27) will use router 192.168.20.33 address as the default gateway address.
Building C Hosts (192.168.20.64/27) will use router 192.168.20.65 address as the default gateway address.
Building D Hosts (192.168.20.96/27) will use router 192.168.20.97 address as the default gateway address.

Benefits of Variable Length Subnet Masking
VLSM Chart

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
20
9.1.5.5 VLSM Chart
Subnetting an IPv4 Network
Benefits of Variable Length Subnet Masking
Address planning can also be accomplished using a variety of tools. One method is to use a VLSM chart to identify which blocks of addresses are available for use and which ones are already assigned. This method helps to prevent assigning addresses that have already been allocated. Using the network from the previous example, the VLSM chart can be used to plan address assignment.
Examining the /27 Subnets
As shown in Figure 1, when using traditional subnetting the first seven address blocks were allocated for LANs and WANs. Recall that this scheme resulted in 8 subnets with 30 usable addresses each (/27). While this scheme worked for the LAN segments, there were many wasted addresses in the WAN segments.
When designing the addressing scheme on a new network, the address blocks can be assigned in a way that minimizes waste and keeps unused blocks of addresses contiguous.
Assigning VLSM Address Blocks
As shown in Figure 2, in order to use the address space more efficiently, /30 subnets are created for WAN links. To keep the unused blocks of addresses together, the last /27 subnet was further subnetted to create the /30 subnets. The first 3 subnets were assigned to WAN links.
.224 /30 host address range 225 to 226: WAN link between R1 and R2
.228 /30 host address range 229 to 230: WAN link between R2 and R3
.232 /30 host address range 233 to 234: WAN link between R3 and R4
.236 /30 host address range 237 to 238: Available to be used
.240 /30 host address range 241 to 242: Available to be used
.244 /30 host address range 245 to 246: Available to be used
.248 /30 host address range 249 to 250: Available to be used
.252 /30 host address range 253 to 254: Available to be used
Designing the addressing scheme in this way leaves 3 unused /27 subnets and 5 unused /30 subnets.

9.2 Addressing Schemes

Structured Design
Planning to Address the Network
Allocation of network addresses should be planned and documented for the purposes of:
Preventing duplication of addresses
Providing and controlling access
Monitoring security and performance
Client addresses – Usually dynamically assigned using the Dynamic Host Configuration Protocol (DHCP).

Sample Network Addressing Plan

Presentation_ID
‹#›
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Confidential
22
9.2.1.1 Planning to Address the Network
9.2.1.2 Assigning Addresses to Devices
Addressing Schemes
Structured Design
As shown in the figure, the allocation of network layer address space within the corporate network needs to be well designed. Address assignment should not be random. There are three primary considerations when planning address allocation.
Preventing Duplication of Addresses – Each host in an internetwork must have a unique address. Without the proper planning and documentation, an address could be assigned to more than one host, resulting in access issues for both hosts.
Providing and Controlling Access – Some hosts, such as servers, provide resources to internal hosts as well as to external hosts. The Layer 3 address assigned to a server can be used to control access to that server. If, however, the address is randomly assigned and not well documented, controlling access is more difficult.
Monitoring Security and Performance – Similarly, the security and performance of network hosts and the network as a whole must be monitored. As part of the monitoring process, network traffic is examined for addresses that are generating or receiving excessive packets. If there is proper planning and documentation of the network addressing, problematic network devices can be easily found.
Assigning Addresses within a Network
Within a network, there are different types of devices, including:
End user clients
Servers and peripherals
Hosts that are accessible from the Internet
Intermediary devices
Gateway
When developing an IP addressing scheme, it is generally recommended to have a set pattern of how addresses are allocated to each type of device. This benefits administrators when adding and removing devices, filtering traffic based on IP, as well as simplifies documentation.
Addressing Schemes
Structured Design
A network addressing plan might include using a different range of addresses within each subnet, for each type of device.
Addresses for Clients
Because of the challenges associated with static address management, end user devices often have addresses dynamically assigned, using Dynamic Host Configuration Protocol (DHCP). DHCP is generally the preferred method of assigning IP addresses to hosts on large networks because it reduces the burden on network support staff and virtually eliminates entry errors.
Another benefit of DHCP is that an address is not permanently assigned to a host but is only leased for a period of time. If we need to change the subnetting scheme of our network, we do not have to statically reassign individual host addresses. With DHCP, we only need to reconfigure the DHCP server with the new subnet information. After this has been done, the hosts only need to automatically renew their IP addresses.
Addresses for Servers and Peripherals
Any network resource, such as a server or a printer, should have a static IP address, as shown in the figure. The client hosts access these resources using the IP addresses of these devices. Therefore, predictable addresses for each these servers and peripherals are necessary.
Servers and peripherals are a concentration point for network traffic. There are many packets sent to and from the IPv4 addresses of these devices. When monitoring network traffic with a tool like Wireshark, a network administrator should be able to rapidly identify these devices. Using a consistent numbering system for these devices makes the identification easier.
Addresses for Hosts that are Accessible from Internet
In most internetworks, only a few devices are accessible by hosts outside of the corporation. For the most part, these devices are usually servers of some type. As with all devices in a network that provide network resources, the IP addresses for these devices should be static.
In the case of servers accessible by the Internet, each of these must have a public space address associated with it. Additionally, variations in the address of one of these devices will make this device inaccessible from the Internet. In many cases, these devices are on a network that is numbered using private addresses. This means that the router or firewall at the perimeter of the network must be configured to translate the internal address of the server into a public address. Because of this additional configuration in the perimeter intermediary device, it is even more important that these devices have a predictable address.
Addresses for Intermediary Devices
Intermediary devices are also a concentration point for network traffic. Almost all traffic within or between networks passes through some form of intermediary device. Therefore, these network devices provide an opportune location for network management, monitoring, and security.
Most intermediary devices are assigned Layer 3 addresses, either for the device management or for their operation. Devices, such as hubs, switches, and wireless access points do not require IPv4 addresses to operate as intermediary devices. However, if we must access these devices as hosts to configure, monitor, or troubleshoot network operation, they must have addresses assigned.
Because we must know how to communicate with intermediary devices, they should have predictable addresses. Therefore, their addresses are typically assigned manually. Additionally, the addresses of these devices should be in a different range within the network block than user device addresses.
Address for the Gateway (Routers and Firewalls)
Unlike the other intermediary devices mentioned, routers and firewall devices have an IP address assigned to each interface. Each interface is in a different network and serves as the gateway for the hosts in that network. Typically, the router interface uses either the lowest or highest address in the network. This assignment should be uniform across all networks in the corporation so that network personnel will always know the gateway of the network no matter which network they are working on.
Router and firewall interfaces are the concentration point for traffic entering and leaving the network. Because the hosts in each network use a router or firewall device interface as the gateway out of the network, many packets flow through these interfaces. Therefore, these devices can play a major role in network security by filtering packets based on source and/or destination IP addresses. Grouping the different types of devices into logical addressing groups makes the assignment and operation of this packet filtering more efficient.

Summary
Subnetting is the process of segmenting a network, by dividing it into multiple smaller network spaces.
Subnetting a subnet, or using VLSM, was designed to avoid wasting addresses..
Size, location, use, and access requirements are all considerations in the address planning process.
IP networks must be tested to verify connectivity and operational performance.

/docProps/thumbnail.jpeg

Related Posts