3Com 3C13636 Configuration Manual page 397

3Com Router 3000 Ethernet Family
Configuration Guide
Table 1-1 IP address classes and ranges
Network
class
A
B
C
D
E
Important features of IP address:
IP addresses are not in a hierarchical structure, which is different from the
structure of telephone number. In other words, IP addresses cannot reflect any
geographical information about the host position.
When a host is connected to two networks at the same time (such as the host used
as a router), it must have two IP addresses with different net-ids corresponding to
two different networks. Such host is called multi-homed host.
According to Internet concept, several LANs connected via transceiver or bridge
are still in the same network, so these LANs have the same net-id.
With respect to IP address, all networks with a net-ids are equal (no matter it is a
small LAN or a huge WAN).
Since 1985, only the net-id of IP address is assigned by NIC, while the host-id is
controlled by the enterprise. The IP address assigned to an enterprise is only a network
ID: net-id. The specific host Ids, the host-ids for respective hosts, shall be assigned by
the enterprise independently, so long as there is no repetition of host IDs within its
network.
If there are many enterprise hosts widely scattered, the host IDs may be further divided
into internal sub-nets to facilitate management. Note that the division of sub-nets is
completely internal to the enterprise itself, and seen from the outside, the enterprise
only has one net-id. When an external packet enters this enterprise network, the
internal router can route according to the sub-net number, and finally reach the
destination host.
The following figure shows the sub-net classification of a Class B IP address, in which a
sub-net mask consists of a string of continuous "1" s and a string of continuous "0" s.
Address range
0.0.0.0 to
127.255.255.255
128.0.0.0 to
191.255.255.255
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
240.0.0.0 to
255.255.255.255
3Com Corporation
1-2
Chapter 2 IP Address Configuration
Description
Network ID with the format of 127.X.Y.Z is
reserved for self-loop test and the packets
sent to this address will not be output to the
line. The packets are processed internally and
regarded as input packets.
—
—
Addresses of class
addresses.
255.255.255.255 is a broadcast address, and
the others are reserved for future use.
D are multicast