Determining the Size of the Network
The first step in designing an IP addressing plan is determining the size of the network to establish how many IP subnets and how many IP addresses are needed on each subnet. To gather this information, answer the following questions:
■ How many locations does the network consist of?: The designer must determine the number and type of locations.
■ How many devices in each location need addresses?: The network designer must determine the number of devices that need to be addressed, including end systems, router interfaces, switches, firewall interfaces, and any other devices.
■ What are the IP addressing requirements for individual locations?: The designer must collect information about which systems will use dynamic addressing, which will use static addresses, and which systems can use private instead of public addresses.
■ What subnet size is appropriate?: Based on the collected information about the number of networks and planned switch deployment, the designer estimates the appropriate subnet size. For example, deploying 48-port switches would mean that subnets with 64 host addresses would be appropriate, assuming one device per port.
Determining the Network Topology
Initially, the designer should acquire a general picture of the network topology; this will help determine the correct information to gather about network size and its relation to the IP addressing plan.
With this general network topology information, the designer determines the number of locations, location types, and their correlations. For example, the network location information for the topology shown in Figure 6-4 is shown in Table 6-1.
Figure 6-4 Sample Network Topology
- Table 6-1 Network Location Information for the Topology in Figure 6-4
|
Location |
Type |
Comments |
|
San Francisco |
Main office |
The central location where the majority of users are located |
|
Denver |
Regional office |
Connects to the San Francisco main office |
|
Houston |
Regional office |
Connects to the San Francisco main office |
|
Location |
Type |
Comments |
|
Remote Office 1 |
Remote office |
Connects to the Denver regional office |
|
Remote Office 2 |
Remote office |
Connects to the Denver regional office |
|
Remote Office S |
Remote office |
Connects to the Houston regional office |
Size of Individual Locations
The network size, in terms of the IP addressing plan, relates to the number of devices and interfaces that need an IP address. To establish the overall network size in a simplistic way, the designer determines the approximate number of workstations, servers, Cisco IP phones, router interfaces, switch management and Layer 3 interfaces, firewall interfaces, and other network devices at each location. This estimate provides the minimum overall number of IP addresses that are needed for the network. Table 6-2 provides the IP address requirements by location for the topology shown in Figure 6-4.
|
Location |
Office Type |
Workstations |
Servers |
Phones |
Router Interfaces |
Switches |
Firewall and Other Device Interfaces |
Reserve |
Total |
|
Francisco |
Main |
600 |
S5 |
600 |
17 |
26 |
12 |
20% |
1290 |
|
Denver |
Regional |
210 |
7 |
210 |
10 |
4 |
0 |
20% |
441 |
|
Houston |
Regional |
155 |
5 |
155 |
10 |
4 |
0 |
20% |
S29 |
|
Remote Office 1 |
Remote |
12 |
1 |
12 |
2 |
1 |
0 |
10% |
28 |
|
Remote Office 2 |
Remote |
15 |
1 |
15 |
S |
1 |
0 |
10% |
S5 |
|
Remote Office S |
Remote |
8 |
1 |
8 |
S |
1 |
0 |
10% |
21 |
|
Total |
1000 |
50 |
1000 |
45 |
S7 |
12 |
2144 |
Some additional addresses should be reserved to allow for seamless potential network growth. The commonly suggested reserve is 20 percent for main and regional offices, and 10 percent for remote offices; however, this can vary from case to case. The designer should carefully discuss future network growth with the organization's representative to obtain a more precise estimate of the required resources.
Continue reading here: Methods of Assigning IP Addresses
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