3Com Switch 8800 Advanced Software V5 Configuration Guide

206 CHAPTER 22: IP ADDRESSING CONFIGURATION
Figure 60 IP address classes
Table 18 describes the address ranges of these five classes. Currently, the first
three classes of IP addresses are used in quantity.
Special Case IP
Addresses
The following IP addresses are for special use, and they cannot be used as host IP
addresses:
■ IP address with an all-zero net ID: Identifies a host on the local network. For
example, IP address 0.0.0.16 indicates the host with a host ID of 16 on the
local network.
■ IP address with an all-zero host ID: Identifies a network.
■ IP address with an all-one host ID: Identifies a directed broadcast address. For
example, a packet with the destination address of 192.168.1.255 will be
broadcasted to all the hosts on the network 192.168.1.0.
Subnetting and Masking Subnetting was developed to address the risk of IP address exhaustion resulting
from fast expansion of the Internet. The idea is to break a network down into
smaller networks called subnets by using some bits of the host-id to create a
subnet-id. To identify the boundary between the host-id and the combination of
net-id and subnet-id, masking is used. (When subnetting is not adopted, a mask
identifies the boundary between the host-id and the host-id.)
Tabl e 18 IP address classes and ranges
Class Address range Description
A 0.0.0.0 to 127.255.255.255 This address is used by a host at
bootstrap when it does not know its
IP address. This address is never a
valid destination address.
Addresses in the format of 127.X.Y.Z
are reserved for the loopback test
purpose. Packets destined to these
addresses are processed locally as
input packets rather than sent to the
link.
B 128.0.0.0 to 191.255.255.255 --
C 192.0.0.0 to 223.255.255.255 --
D 224.0.0.0 to 239.255.255.255 Multicast address
E 240.0.0.0 to 247.255.255.255 Reserved address
0 Net-id Host-id
1 Net-id Host-id0
1
Net-id Host-id
01
1 Multicast address011
1 Reserved111
Class A
07152331
Class B
Class C
Class D
Class E