Networking Concepts

Subnetworks

The following rules apply when choosing a subnet mask and an IP address:

•Although any bits in the node address can be used as the subnet mask, Hewlett-Packard recommends aligning the subnet mask along byte boundaries, adjacent to the network number.

•Although standards allow subnets on the same network to have different subnet masks, Hewlett-Packard recommends that you assign the same subnet mask to all subnets on a network.

•Do not assign an IP address where the network address and/or node address bits are all off (all 0s) or all on (all 1s). Likewise, the subnet address bits cannot be all 0s or all 1s.

To determine the subnet mask, you first need to estimate the number of networks required and the number of nodes on each subnet. Allow enough bits for both nodes and subnets, as described in example 1.

Example 1

Assume you are choosing a subnet mask for a class C network (three

 

bytes for network address, one byte for node address), and you need

 

four subnets with up to 30 nodes on each subnet. You will need to

 

reserve three bits for the subnet address (remember, all 0s and all 1s

 

cannot be used) and the remaining five bits for the node numbers as

 

shown in Figure 2-1.

Figure 2-1

Class C Address with Subnet Number

 

The 30 nodes per subnet will require at least five bits of the node

 

portion of the IP address (30 <32, and 32=25, therefore you need 5 bits).

 

This leaves three bits remaining in the node portion of the IP address

 

for use as the subnet identifier. Subnet parts of all 0’s or all 1’s are not

 

recommended because they can be confused with broadcast addresses.

 

Therefore, you can have up to six subnets (23 –2=6) when three bits are

 

used for the subnet identifier.

Example 2

An IP address on a class B network with an 8-bit subnet mask

 

separates as shown in Figure 2-2.

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Chapter 2