编写网界青年“健康代码”——“网界青年成长计划”之“青咖说”身心健康专场（二）活动在京举行

An IP address refers to an Internet Protocol Address, which is abbreviated as IP Address. An IP address is a uniform address format provided by the IP protocol, assigning a logical address to each network and host on the Internet, thereby masking the differences in physical addresses.

IP addresses are used to number computers (or smartphones) on the Internet. A commonly seen scenario is that every PC connected to the network needs an IP address to communicate properly. We can liken a "personal computer" to a "telephone," in which case the "IP address" is equivalent to the "telephone number." The routers in the Internet are akin to the "program-controlled switches" in a telephone exchange.

The concept of IP address: An IP address is the basis for TCP/IP communication (Internet Interconnect Protocol), and every computer connected to the network must have an IP address. The IP addresses we currently use, IPv4, are 32 bits long, usually divided into 4 "8-bit binary numbers" (i.e., 4 bytes). IP addresses are commonly represented in "dotted decimal" format as (a.b.c.d), where a, b, c, d are decimal integers between 0 ~ 255. For example, the dotted decimal IP address (100.4.5.6) actually represents a 32-bit binary number (01100100.00000100.00000101.00000110).

Common IP addresses are divided into two main categories: IPv4 and IPv6. IPv4 uses a 32-bit address length, consisting of 4 segments of numbers, each segment not exceeding 255. IPv6 uses a 128-bit address length.

The format of an IP address: An IP address = Network address + Host address, or IP address = Network address + Subnet address + Host address. A simple IP address actually contains two important pieces of information: the network address and the host address.

Let's take the IPv4 address 192.168.1.100 as an example. In this case, this IPv4 address can be broken down as follows:
· Network address: 192.168.1 (the first three segments of the IP address)
· Host address: 100 (the last segment)

If subnetting is involved, the structure of an IPv4 address can be more complex. For example, if there is a subnet with a subnet mask of 255.255.255.0, then this IPv4 address can be broken down as follows:
· Network address: 192.168.1 (the first three segments of the IP address)
· Subnet address: 0 (determined by the subnet mask)
· Host address: 100 (the last segment)

The above is an example of a simple IPv4 address format. Actual networks might have more complex subnet divisions and longer IP addresses. The representation of IPv6 addresses is different, but the basic principle is similar.

IP addresses are divided into 5 types based on the network ID: A/B/C/D/E.

1. Class A: This class is used for very large networks. IP addresses in the first 8-bit byte ranging from 1 to 126 belong to this group.
2. Class B: This class is used for medium-sized networks. IP addresses in the first 8-bit byte ranging from 128 to 191 belong to this group.
3. Class C: This class is used for small to medium-sized networks. IP addresses in the first 8-bit byte ranging from 192 to 223 belong to this group.
4. Class D: This class is used for multicast, with the first three bits set to 1 and the fourth bit set to 0.
5. Class E: This class is only used for experimental purposes. It is very similar to Class D, with the first three bits set to 1, but the significant difference is that its fourth bit is also set to 1.