International SMS Class A or Class B

Class A IP address refers to the four segments of an IP address, the first segment is the network number, and the other three segments are the numbers of the local computer. If the IP address is expressed in binary, the Class A IP address consists of a network address of 1 byte and a host address of 3 bytes, and the highest bit of the network address must be "0". Among the Class A IP addresses, the length of the network identifier is 7 bits, and the length of the host identifier is 24 bits. The number of Class A network addresses is small, which can be used for large networks with more than160,000 hosts.

2. Class B IP address

Class B IP address means that among the four parts of IP address, the first two parts are network numbers and the remaining two parts are local computer numbers. If the IP address is expressed in binary, the Class B IP address consists of a 2-byte network address and a 2-byte host address, and the highest bit of the network address must be "10". In class B IP address, the length of network identification is 14 bits, and the length of host identification is 16 bits. Class B network addresses are suitable for medium-sized networks, and each network can accommodate more than 60,000 computers.

3. Class C IP address

Class C IP address refers to the four sections of an IP address, the first three sections are the network number, and the remaining section is the number of the local computer. If the IP address is expressed in binary, the Class C IP address consists of a 3-byte network address and a 1 byte host address, and the highest bit of the network address must be "1 10". The length of network identification in class C IP address is 2 1 bit, and the length of host identification is 8 bits. Class C network has many addresses and is suitable for small-scale local area networks. Each network can only accommodate 254 computers at most.

In addition to the above three types of IP addresses, there are several special types of IP addresses. According to the TCP/IP protocol, any address whose first byte begins with "lll0" is called a multicast address. So any IP address with the first byte greater than 223 and less than 240 is a multicast address; An address in which every byte in the IP address is 0 ("0.0.0.0") corresponds to the current host; The IP address ("255.255.255.255") in which every byte in the IP address is 1 is the broadcast address of the current subnet; All IP addresses with "llll0" are reserved for special use in the future; IP address cannot start with decimal "127", and the number 127 in this address? 27. 1. 1. 1 is used for loop testing, and the first 6 groups of the network ID cannot all be set to "0", indicating the local network.

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Three. IP addressing rules

1. network addressing rules

A. the network address must be unique.

B the network logo cannot start with the number 127. In Class A addresses, 127 is reserved for the internal loopback function.

The first byte of the c network identifier cannot be 255. The number 255 is used as the broadcast address.

D the first byte of the network identifier cannot be "0", which means that the address is a local host and cannot be transmitted.

2. Host addressing rules

Host identification must be unique in the same network.

B all bits of the host identifier cannot be "1". If all bits are "1", the address of the host is the broadcast address, not the address of the host.

C all bits of the host identifier cannot be "0". If all bits are "0", it means "only this network" and there is no host on this network.

Four. Overview of IP subnet mask

The concept of 1. subnet mask

A subnet mask is a 32-bit address used to mask a part of an IP address to distinguish the network identification from the host identification, and to indicate whether the IP address is on a local area network or a remote network.

2. Determine the number of subnet masks

The number of bits used for the subnet mask depends on the number of possible subnets and the number of hosts per subnet. Before defining the subnet mask, it is necessary to find out the number of subnets and hosts used initially.

To define a subnet mask:

A. determine which group addresses we use. For example, the network number we applied for is "2 10.73.a.b", the network address is a class C IP address, the network identifier is "2 10.73" and the host identifier is "a.b".

B, according to the number of subnets we need now and the number of subnets we may expand to in the future, define the subnet mask "1" with some bits of the host. 2 subnets, and 16 may be needed in the future. Use the first four bits of the third byte to determine the subnet mask. The first four bits are all set to "1", that is, the third byte is "110000", which is temporarily called the new binary subnet mask.

C all bits corresponding to the initial network are set to "1", that is, the first two bytes are set to "1" and the fourth byte is set to "0". Then the discontinuous binary form of the subnet mask is: "1111111/.

D, convert this number into discontinuous decimal form: "255.255.240.0"

This number is the subnet mask of this network.

Mark of 3.3. IP mask

I. Subnet-free Representation

For an IP address without a subnet, it can be written as a mask with a host number of 0. For example, the IP address is 2 10.73. 140.5, and the mask is 255.255.255.0. You can also default to the mask and write only the IP address.

B, using the method of subnet marking

When there are subnets, they must appear in pairs. Take a class C address as an example.

The first three bytes in the 1.IP address indicate the network number, the last byte indicates the subnet number and host number, and also indicates whether the two IP addresses belong to a network segment. If they belong to the same network interval, the information exchange between these two addresses will not go through the router. If they are not in the same network interval, that is, the subnet numbers are different, the information exchange between the two addresses will be carried out through the router. For example, for a host with IP address 2 10.73. 140.5, its host identifier is 0000 10 1, and for an IP address 210.73.140./

2. The mask is used to indicate how many subnets there are, but the number of subnets can only be expressed as a range. You can't say for sure how many subnets there are. The mask does not specify a specific subnet number, but it has a subnet mask format (for class C addresses): the first few digits of the host identifier are the subnet number, followed by the host, and all zeros are written.

Verb (abbreviation of verb) Other matters of intellectual property rights.

1. Generally, Internet information centers assign IP addresses according to the network, so only when network addresses are involved can they use the expressions of Class A, Class B and Class C;

2. When assigning network addresses, the network identification is fixed, while the computer identification can be changed within a certain range. The following are three forms of network addresses:

Class A address: 73.0.0.0

Class b address: 160. 153.0.0.

Class C address: 2 10.73. 140.0.

Each of the above zeros can vary from 0 to 255.

3. Because the first three digits of the IP address have already decided what kind of network the IP address belongs to, class A network address can no longer be divided into class B IP addresses, and class B IP addresses can no longer be divided into class C IP addresses.

4. When talking about the specific computer IP address, it is not appropriate to use the expressions of class A, class B and class C, but it can be said which class A, class B and class C network the host address belongs to.

Through the above study, everyone must know something about IP addresses. With an IP address, people can send emails and get other information on the Internet, such as WWW service, BBS service, FTP service and so on.