3Com 4210 PWR Configuration Manual page 90

88 C 7: IP A
HAPTER DDRESSING
Special Case IP
Addresses
Subnetting and Masking
C
ONFIGURATION
Table 48 IP address classes and ranges
Class
A 0.0.0.0 to 127.255.255.255
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
E 240.0.0.0 to 255.255.255.255
The following IP addresses are for special use, and they cannot be used as host IP
addresses:
IP address with an all-zeros 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-zeros host ID: Identifies a network.
■
IP address with an all-ones 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 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.
Each subnet mask comprises 32 bits related to the corresponding bits in an IP
address. In a subnet mask, the section containing consecutive ones identifies the
combination of net ID and subnet ID whereas the section containing consecutive
zeros identifies the host ID.
Figure 35 shows how a Class B network is subnetted.
Figure 35 Subnet a Class B network
0
Class B address
1 0
Mask
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Subnetting
Mask
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0
Address range
7 15
Net-id
Net-id Subnet-id
Description
Address 0.0.0.0 means this host no
this network. 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 starting with 127 are
reserved for loopback test. Packets
destined to these addresses are
processed locally as input packets
rather than sent to the link.
--
--
Multicast address.
Reserved for future use except for
the broadcast address
255.255.255.255.
23 31
Host-id
Host-id