Module 2 - Design the Network Structure

Section 4 - Design a Network-Layer Addressing and Naming Model

Section Objectives

• Propose an addressing model for the customer's areas, networks, subnetworks, and end stations that meets scalability requirements.

• Propose a plan for configuring addresses.

• Propose a naming scheme for servers, routers, and user stations.

Time Required to Complete This Section

Completing This Section

1. Study the reading assignment.
2. Click on any links that you see in the reading assignment and review the information that appears.
3. Review any tables and job aids that appear in the reading assignment.
4. Review the case studies at the end of this section.
5. Complete the questions in each case study.
6. Review the answers provided by our internetworking experts.

Resources Required to Complete This Section

• Access to the World Wide Web and Cisco's Web site
• A downloaded, printed copy of this section
• Paper and pencil

Reading Assignment 1

Steps for Designing Network-Layer Addressing and Naming

To design network-layer addressing and naming, follow these steps:

     1. Design a hierarchy for addressing autonomous systems, areas, networks, subnetworks, and end stations.

     2. Design route summarization (aggregation).

     3. Design a plan for distributing administrative authority for addressing and naming at the lower levels of
         the hierarchy.

     4. Design a method for mapping geographical locations to network numbers.

     5. Develop a plan for identifying special stations such as routers and servers with specific node IDs.

     6. Develop a plan for configuring user-station addresses (dynamic if possible). Use the Bootstrap Protocol
         (BOOTP) or the
         newer DHCP.

     7. If necessary, develop a plan for using gateways to map private addresses to external addresses.

• Network Address Translation (NAT) if the following private addresses, as specified in RFC 1918, are used:
 
• 10.0.0.0 to 10.255.255.255
• 172.16.0.0 to 172.31.255.255
• 192.168.0.0 to 192.168.255.255

•  Names should be meaningful to facilitate troubleshooting.
 
• To name devices in IP environments, install and configure DNS servers. Use Cisco's DNS/DHCP Manager to synchronize DNS names with dynamically assigned IP addresses.

Designing IP Addressing to Facilitate Route Summarization

Route summarization reduces the number of routes in the routing table, the routing update traffic, and overall router overhead. Reducing routing update traffic can be very important on low-speed lines. If the Internet had not adapted route summarization by standardizing on classless interdomain routing (CIDR), it would not have survived.

The telephone architecture has handled prefix routing for many years. A telephone switch in Michigan need not to know how to reach a specific line in Oregon. It just needs to recognize that the call is not local. A long-distance carrier needs to recognize that 541 is for Oregon but does not need to know the details of how to reach the specific line in Oregon.

Prefix routing is not new in the IP environment either. A router needs to know only how to reach the next hop. It need not to know the details of how to reach an end node that is not local.

Much as in the telephone example, IP routers make hierarchical decisions. An IP address comprises prefix part and a host part. Routers use the prefix to determine the path for a destination address that is not local. The host part is used to reach local hosts.

A prefix identifies a block of host numbers and is used for routing to that block. According to RFC 1518, a prefix is "an IP address and some indication of the leftmost contiguous significant bits within that address." The indication of the leftmost contiguous bits has traditionally been done with an indication of the address class and a subnet mask. More recently, a length indication has followed a network number and slash, for example, 192.10.168.0/21.

Traditional classfull routing does not transmit any information about the prefix length. Hosts and routers calculate the prefix length by looking at the first few bits of an address to determine whether it is one of the following:
 

Class A	first bit = 1	prefix is 8 bits
Class B	first 2 bits = 10	prefix is 16 bits
Class C	first 3 bits = 110	prefix is 24 bits

 

VLSM relies on providing prefix length information explicitly with each use of an address. The length of the prefix is evaluated independently at each place it is used.

The ability to have a different prefix length at different points supports more efficient use of the IP address space and reduces routing traffic. Big subnets and small subnets are allowed. If the small subnets are grouped, routing information can be summarized (aggregated) into a small number of routing table entries.

For summarization to work correctly, the following requirements must be met:

• Multiple IP addresses must share the same leftmost bits.

• Routers must base their routing decisions on a 32-bit IP address and prefix length that can be up to 32 bits.

• Routing protocols must carry the prefix length with the 32-bit IP address.

• 192.108.168.0
• 192.108.169.0
• 192.108.170.0
• 192.108.171.0
• 192.108.172.0
• 192.108.173.0
• 192.108.174.0
• 192.108.175.0

By advertising this one route, the router is saying, "Route packets to me if the destination has the first 21 bits set to 192.108.168."

In binary, the 21 bits are as follows (x is a wildcard - it does not matter how those bits are set):

• 1100 0000 (first octet = 192)
• 0110 1100 (second octet = 108)
• 1010 1xxx (third octet starts with 10101, which is 168 in decimal)
• xxxx xxxx (fourth octet can be anything)

IPX Addressing

IPX Addressing Guidelines

A Novell IPX address:
 
• Uses a two-part address, which includes the network number and the node number.
• Has 80 bits: 32 bits for the network number and 48 bits for the node number.
• Contains the MAC address of an interface.
• Can be up to 16 hexadecimal digits in length.

 Example IPX Address

In the following example:
 
• The main IPX network has an address of 4a1d. Other IPX networks shown are 2c and 3f.
• The IPX node number is 12 hexadecimal digits in length. This number is usually the MAC address obtained from a router interface that has a MAC address.
• The example features the IPX node 0000.0c56.de33. Another node address is 0000.0c56.de34.
• The same node number appears for both E0 and S0. Serial interfaces do not have MAC addresses, so Novell IPX obtained this node number for S0 by using the MAC address from E0.
 

 
Each interface retains its own address. The use of the MAC address in the logical address eliminates the need for an Address Resolution Protocol (ARP).

You must use a valid IPX network address when you configure the Cisco router. Because the Novell NetWare networks are likely to be already established with IPX addresses, determine the IPX address from these already established networks. The IPX network address refers to the "wire"; all routers on the same wire must share the same IPX network address.

Selecting IPX Addresses

The first and recommended way to find out which address to use is to ask the NetWare administrator. Make sure that the NetWare administrator specifies the IPX network address for the same network where you want to enable IPX on your Cisco router. The Cisco router must use the same network as the NetWare file server (or other source of the address) specified by the NetWare administrator.

If you cannot obtain an IPX address to use from the NetWare administrator, you can get the neighbor’s IPX address directly from a neighbor router. Use any one of the following methods to obtain the IPX address:
 

• If the neighbor router is another Cisco router, you can use a Cisco IOS command to show Cisco Discovery Protocol (CDP) neighbor details.

• You can Telnet to the neighbor router, enter the appropriate mode, then display the running configuration on the neighbor.

• If the neighbor router is not a Cisco router (is a NetWare PC-based router, or a NetWare file server), you may be able to attach or log in and use a NetWare utility config to determine the address.

• On the Cisco router, you must use the same IPX network address as the address that already exists on that network.

IP Addressing

• Enterprise reorganization
• Physical moves of equipment
• New strategic relationships
• Changes in ISP
• New applications
• The needs of global Internet connectivity
• Route summarization implementation

Howard Berkowitz has also written a very helpful draft RFC that outlines the steps for router renumbering titled Router Renumbering Guide rfc2072.txt.

Case Studies

In this section, you will implement an addressing scheme for each case study.

Read each case study and complete the questions that follow. Keep in mind that there are potentially several correct answers to each question.

When you complete each question, you can refer to the solutions provided by our internetworking experts. The case studies and solutions will help prepare you for the Sylvan exam following the course.

In this section, you will review the following case studies:

1. Virtual University
2. CareTaker Publications, a publishing company
3. PH Network Services Corporation, a health care company
4. Pretty Paper Ltd., a European wall covering company
5. Jones, Jones, & Jones, an international law firm

Case Study: Virtual University

Virtual University has decided to eliminate AppleTalk and use only IP. The university will use the IP network number 172.16.0.0. The university has a North Campus, Central Campus, and South Campus. Each campus has 40 networks and each network has 150 nodes. The network administrators expect to expand to 60 networks and 200 nodes per network within the next five years.

Because of its AppleTalk heritage, Virtual University needs a simple addressing solution with very little end-node configuration. Despite its AppleTalk heritage, Virtual University has some knowledgeable IP gurus who have specified that the addressing scheme must be conducive to route summarization (aggregation).
 
     1. Design a model for dividing up Virtual University's IP address space that will meet the university's current
         needs and needs for the next five years. Describe your model here.

Case Study: CareTaker Publications

You might find it useful to refer to your topology diagram for CareTaker Publications in Section 3.

The parent corporation told CareTaker that CareTaker will be protected from Internet "hackers" with the firewall at the corporate facilities. Because of the limited number of IP addresses available, the parent corporation has informed CareTaker that it will receive one Class C address and is to implement "Big Internet" addressing within the confines of CareTaker.

     1. Design a model for CareTaker's IP address space that will meet the current needs and needs for the next
         five years. Describe your model here.

Now that you have completed the exercise, click here to view the solutions provided by our internetworking design experts.

Case Study: PH Network Services Corporation

You might find it useful to refer to your topology diagram created for PH Network Services Corporation in Section 3.
 
     1. The hospital system has an existing IP network with its own IP addresses. The hospital will be able to
         assign two Class C addresses to the PH Network: one for the WAN (202.12.27.0) and one for PH’s
         internal use (202.12.28.0). Describe your IP addressing plans for the implementation of PH’s network.
         You will use a Class C mask of 255.255.255.0 for the PH LAN. What mask will you use for the WAN?

Now that you have completed the exercise, click here to view the solutions provided by our internetworking design experts.

Case Study: Pretty Paper Ltd.

You might find it useful to refer to your topology diagram created for Pretty Paper in Section 3.
 
     1. The network administrator has been using the Class B IP address of 199.151.000.000. He does not
         know where he got it but he is sure that Pretty Paper does not own it. What are your recommendations
         for an IP address allocation/assignment procedure?

Case Study: Jones, Jones, & Jones

You might find it useful to refer to your topology diagram created for Mr. Jones in Section 3.
 
     1. Describe your IP addressing plans for implementation of your proposed system design.

Now that you have completed the exercise, click here to view the solutions provided by our internetworking design experts.

If you have completed this section, click here to advance to Module 2, Section 5.