3Com MSR 50 Series, Руководство по настройке

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624 C
HAPTER
40: IP A
DDRESSING
C
ONFIGURATION
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 In 1980s, 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.)
Each subnet mask comprises 32 bits related to the corresponding bits in an IP
address. In a subnet mask, the part containing consecutive ones identifies the
combination of net-id and subnet-id whereas the part containing consecutive
zeros identifies the host-id.
Figure 186 shows how a Class B network is subnetted.
Figure 186 Subnet a Class B network
Table 34 IP address classes and ranges
Class Address range Description
A 0.0.0.0 to 127.255.255.255 The IP address 0.0.0.0 is used by a host
at bootstrap for temporary
communication. 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.
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 255.255.255.255 Reserved for future use except for the
broadcast address 255.255.255.255.
1
Net-id Host-id
0
Class B address
0 7 15 23 31
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
Net-id Host-id
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
Subnet-id