3Com Switch 7754, Руководство по настройке

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128 C
HAPTER
15: IP A
DDRESS
C
ONFIGURATION
Subnet and Mask The traditional IP address classification method wastes IP addresses greatly. In
order to make full use of the available IP addresses, the concepts of mask and
subnet were introduced.
A mask is a 32-bit number corresponding to an IP address. The number consists of
1s and 0s. A mask is defined as follows: the bits of the network number and
subnet number are set to 1, and the bits of the host number are set to 0. The
mask divides the IP address into two parts: subnet address and host address. In an
IP address, the part corresponding to the “1” bits in the mask is the subnet
Table 73 Classes and ranges of IP addresses
Network
type Address
range
IP network
range Description
A 0.0.0.0 to
127.255.255.255
1.0.0.0 to
126.0.0.0
■
An IP address with all 0s host ID is a
network address and is used for
network routing.
■
An IP address with all 1s host ID is a
broadcast address and is used for
broadcast to all hosts on the
network.
■
The IP address 0.0.0.0 is used by
hosts when they are booted but is
not used afterward.
■
An IP address with all 0s network ID
represents a specific host on the
local network and can be used as a
source address but cannot be used
as a destination address.
■
All the IP addresses in the format of
127.X.Y.Z are reserved for loopback
test and the packets sent to these
addresses will not be output to
lines; instead, they are processed
internally and regarded as incoming
packets.
B 128.0.0.0 to
191.255.255.255
128.0.0.0 to
191.255.0.0
■
An IP address with all 0s host ID is a
network address and is used for
network routing.
■
An IP address with all 1s host ID is a
broadcast address and is used for
broadcast to all hosts on the
network.
C 192.0.0.0 to
223.255.255.255
192.0.0.0 to
223.255.255.0
■
An IP address with all 0s host ID is a
network address and is used for
network routing.
■
An IP address with all 1s host ID is a
broadcast address and is used for
broadcast to all hosts on the
network.
D 224.0.0.0 to
239.255.255.255
None Class D addresses are multicast
addresses.
E 240.0.0.0 to
255.255.255.254
None These IP addresses are reserved for
future use.
Others 255.255.255.255 255.255.255.25
5
255.255.255.255 is used as a LAN
broadcast address.