3Com 3C13636 Configuration Manual page 398

Router 3000 ethernet family

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3Com Router 3000 Ethernet Family

Configuration Guide

The 1s corresponds to the network ID field and the sub-net number field, while the 0s

correspond to the host ID field.

Class B address

number field

Figure 1-2 Sub-net classification of IP address

Classification of one more sub-net number field is at a price. For example, an IP

address of class B originally consists of 65534 (2

6-bit-long sub-net field is classified, there may be at most 64 sub-nets. Each sub-net

has 10-bit host ID, i.e., each sub-net has 1022 host IDs at most. There are 64 x

1022=65408 host IDs in total, 126 less than the sum before sub-net classification.

If there is no sub-net division in an enterprise, then its sub-net mask is the default value

and the length of "1" indicates the net-id length. Therefore, for IP addresses of classes

A, B and C, the default values of corresponding sub-net mask are 255.0.0.0,

255.255.0.0 and 255.255.255.0 respectively.

A router used to connect multiple sub-nets together will have multiple sub-net IP

The IP addresses mentioned above cannot be directly used in communication,

An IP address is only an address of a host in the network layer. To send the data

packets transmitted through the network layer to the destination host, physical

address of the host is required. So the IP address must be first resolved into a

physical address.

IP address is hard to remember, but a host domain name will be much easier to

remember and is also more popular. So the host domain name must also be

resolved into an IP address.

The following figure illustrates relations among host name, IP address and physical

3Com Corporation

Chapter 2 IP Address Configuration

Local distribution

111111 00 00000000

-2) host IDs. However, after a

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3c13636-us - router 3036 3000 series