Subnet And Mask - 3Com 8800 Configuration Manual

3Com Switch 8800 Configuration Guide
Network
class
D
E
Other
addresses

11.1.2 Subnet and Mask

Nowadays, with rapid development of the Internet, IP addresses are depleting very fast.
The traditional IP address allocation method wastes IP addresses greatly. In order to
make full use of the available IP addresses, the concept of mask and subnet is
proposed.
A mask is a 32-bit number corresponding to an IP address. The number consists of 1s
and 0s. Principally, these 1s and 0s can be combined randomly. However, the first
consecutive bits are set to 1s when you design a mask. The mask divides the IP
address into two parts: subnet address and host address. The part of IP address that
corresponds to the bits 1s in the mask indicates the subnet address and the other part
of IP address indicate the host address. If there is no subnet division, then its subnet
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 subnet mask
are 255.0.0.0, 255.255.0.0 and 255.255.255.0 respectively.
The mask can be used to divide a Class A network containing more than 16,000,000
hosts or a Class B network containing more than 60,000 hosts into multiple small
networks. Each small network is called a subnet. For example, for the Class B network
address 138.38.0.0, the mask 255.255.224.0 can be used to divide the network into
eight subnets: 138.38.0.0, 138.38.32.0, 138.38.64.0, 138.38.96.0, 138.38.128.0,
IP network
Address
range
range
available
224.0.0.0
to
None
239.255.2
55.255
240.0.0.0
to
None
255.255.2
55.254
255.255.2 255.255.255.2
55.255 55
11-3
Chapter 11 IP Address Configuration
Note
Addresses of class D are multicast
addresses, among which:
IP address 224.0.0.0 is reserved
and will not be allocated. Those
from 224.0.0.1 to 224.0.0.255 are
reserved for routing protocols and
other protocols that are used to
discover and maintain routes.
Those from
239.255.255.255 are used for local
multicast management.
Those from 224.0.0.255
238.255.255.255 are for users.
The addresses are reserved for future
use.
255.255.255.255 is used as a Local
Area Network (LAN)
address.
239.0.0.0 to
to
broadcast