Netopia 4752 2a4na 用户手册
C-2 Administration Guide
IP addresses are maintained and assigned by the InterNIC, a quasi-governmental organization now increasingly
under the auspices of private industr y.
under the auspices of private industr y.
Note: It’s ver y common for an organization to obtain an IP address from a third par ty, usually an Internet
ser vice provider (ISP). ISPs usually issue an IP address when they are contracted to provide Internet access
ser vices.
ser vice provider (ISP). ISPs usually issue an IP address when they are contracted to provide Internet access
ser vices.
The InterNIC (the NIC stands for Network Information Center) divides IP addresses into several classes.
Classes A, B, and C are assigned to organizations that request addresses. In Class A networks, the first byte of
an IP address is reser ved for the network por tion of the address. Class B networks reser ve the first two bytes
of an IP address for the network address. Class C networks reser ve the first three bytes of an IP address for the
network address. In all cases, a network manager can decide to use subnetting to assign even more bits to the
network por tion of the IP address, but never less than the class requires. The following section gives more
information on subnetting.
Classes A, B, and C are assigned to organizations that request addresses. In Class A networks, the first byte of
an IP address is reser ved for the network por tion of the address. Class B networks reser ve the first two bytes
of an IP address for the network address. Class C networks reser ve the first three bytes of an IP address for the
network address. In all cases, a network manager can decide to use subnetting to assign even more bits to the
network por tion of the IP address, but never less than the class requires. The following section gives more
information on subnetting.
Class A networks have a small number of possible network numbers, but a large number of possible host
numbers. Conversely, Class C networks have a small number of possible host numbers, but a large number of
possible network numbers. Thus, the InterNIC assigns Class A addresses to large organizations that have ver y
large numbers of IP hosts, while smaller organizations, with fewer hosts, get Class B or Class C addresses. You
can tell the various classes apar t by the value of the first (or high-order) byte. Class A networks use values from
1 to 127, Class B networks use values from 128 to 191, and Class C networks use values from 192 to 223.
The following table summarizes some of the differences between Class A, B, and C networks.
numbers. Conversely, Class C networks have a small number of possible host numbers, but a large number of
possible network numbers. Thus, the InterNIC assigns Class A addresses to large organizations that have ver y
large numbers of IP hosts, while smaller organizations, with fewer hosts, get Class B or Class C addresses. You
can tell the various classes apar t by the value of the first (or high-order) byte. Class A networks use values from
1 to 127, Class B networks use values from 128 to 191, and Class C networks use values from 192 to 223.
The following table summarizes some of the differences between Class A, B, and C networks.
Subnets and subnet masks
Often an entire organization is assigned only one IP network number. If the organization has several IP networks
connected together with IP routers, the network manager can use subnetting to distinguish between these
networks, even though they all use the same network number. Each physical network becomes a subnet with a
unique subnet number.
connected together with IP routers, the network manager can use subnetting to distinguish between these
networks, even though they all use the same network number. Each physical network becomes a subnet with a
unique subnet number.
Subnet numbers appear within IP addresses, along with network numbers and host numbers. Since an IP
address is always 32 bits long, using subnet numbers means either the network number or the host numbers
must use fewer bits in order to leave room for the subnet numbers. Since the InterNIC assigns the network
number proper, it should not change, so the subnet numbers must be created out of bits that would other wise
be par t of the host numbers.
address is always 32 bits long, using subnet numbers means either the network number or the host numbers
must use fewer bits in order to leave room for the subnet numbers. Since the InterNIC assigns the network
number proper, it should not change, so the subnet numbers must be created out of bits that would other wise
be par t of the host numbers.
Class
First byte
Number of
networks
possible per
class
Number of
hosts
possible per
network
Format of address
(without subnetting)
Example
A
1–127
127
16,777,214
net.host.host.host
97.3.14.250
B
128–191
16,384
65,534
net.net.host.host
140.100.10.11
C
192–223
2,097,152
254
net.net.net.host
197.204.13.7