IP, subnet mask, gateway, default gateway

ip

ip Address = network address + The host address （ also called ： Network number and host number ）, We call the host with the same network number as the host in the local network , Hosts with different network numbers are called hosts in the remote network . Hosts in the local network can communicate with each other directly , In order to communicate with each other, the hosts in the remote network must pass through the local gateway （Gateway） To deliver forwarding data .

ip The main addresses are A、B、C、D、E Five types of address ：

• A class ： from 1 Byte network address and 3 Byte host address composition , The highest bit of the network address must be “0”（ All network addresses are 0 The address of is reserved for use ）. The address range is 1.0.0.0 To 127.255.255.255, It is mainly allocated to large-scale networks with a large number of hosts and a small number of LAN networks ;
• B class ： from 2 Byte network address and 2 Byte host address composition , The top two digits of the network address must be “10”. The address range is 128.0.0.0 To 191.255.255.255, Generally used in large international companies and government agencies ;
• C class ： from 3 Byte network address and 1 Byte host address composition , The highest bit of the network address must be “110”. The address range is 192.0.0.0 To 223.255.255.255, For general small company campus network research institutions, etc ;
• D class ： The first byte consists of “1110” Start . All zeros (“0．0．0．0”) The address corresponds to the current host . whole “1” Of IP Address (“255．255．255．255”) Is the broadcast address of the current subnet . The address range is 224.0.0.0 To 239.255.255.255, Also known as broadcast address , So any first byte is greater than 223 Less than 240 Of IP The address is the multicast address .
• E class ： from “11110” Start , The address range is 240.0.0.0 To 255.255.255.255, For the time being .

network address

Calculating the network address is to determine whether the computers in the network are on the same network , You can get through , You can't get through without , Need the help of local gateway .

take IP The address and subnet mask are all converted to binary form , Then the result of the phase by phase sum of the two is the network address .

IP Decimal system ：192.168.1.1
IP Binary system ：11000000 10101000 00000001 00000001

 Subnet mask decimal ：255.255.255.0
 Subnet mask binary ：11111111 11111111 11111111 00000000

 network address  = IP &  Subnet mask 
 Network address binary ：11000000 10101000 00000001 00000000
 Network address decimal ：192.168.1.0

The host address

IP The result of not matching the subnet mask is the host address .

IP Decimal system ：192.168.1.1
IP Binary system ：11000000 10101000 00000001 00000001

 Subnet mask decimal ：255.255.255.0
 Subnet mask binary ：11111111 11111111 11111111 00000000

 The host address  = IP & (~ Subnet mask )
 Host address binary ：00000000 00000000 00000000 00000001
 Host address decimal ：0.0.0.1

Subnet mask

The representation of subnet mask

（1） Dotted decimal notation
Binary to decimal , Every time 8 The places are separated by dots .

for example ： Subnet mask binary 11111111.11111111.11111111.00000000, Expressed as 255.255.255.0

（2）CIDR slash notation
IP Address /n

example 1：192.168.1.100/24, Its subnet mask is expressed as 255.255.255.0, Binary representation as 11111111.11111111.11111111.00000000

example 2：172.16.198.12/20, Its subnet mask is expressed as 255.255.240.0, Binary representation as 11111111.11111111.11110000.00000000

It's not hard to find out , example 1 The Communist Party of China has 24 individual １, example 2 The Communist Party of China has 20 individual １, therefore n It's like this . Operator, ISP This method is often used to assign... To customers IP Address .

notes ：n by 1 To 32 The number of , Indicates the length of the network number in the subnet mask , adopt n Determine the number of hosts in the subnet =2^(32-n)-2（-2 Why ： All hosts are 0 Indicates the network address of the network , All hosts are 1 Indicates the broadcast address of the network , These are two special addresses ）.

Why use a subnet mask ？

Said before , The subnet mask can be separated IP Network address and host address in the address , Then why separate ？ Because the two hosts need to communicate , First of all, it is necessary to judge whether they are in the same network segment , Whether the network address is the same . If the same , Then you can send the packet directly to the target host , Otherwise, the routing gateway is required to forward the data packet to the destination .

It's easy to understand ：A The host should be connected with B Host communication ,A and B Respective IP Address and A The subnet mask of the host And And operation , The visible result ：

• If the results are the same , It means that the two hosts are in the same network segment , such A Can pass ARP Broadcast discovery B Of MAC Address ,B It can also be found A Of MAC Address to achieve normal communication .
• If the results are different ,ARP The broadcast will end at the local gateway , Now A Will send to B The packet is sent to the local gateway first , The gateway is then based on B The host IP Address to query the routing table , Then continue to deliver and forward the packet , Finally sent to the destination B.

Classification of subnet masks

（1） The default subnet mask
Also known as the default subnet mask , That is, there is no subnet , The corresponding network number is set to 1 , All the host numbers are set 0 .

Not divided into subnets IP Address ： network number ＋ Host number , be ：

• A Class network default subnet mask ： 255.0.0.0, use CIDR Expressed as /8
• B Class network default subnet mask ： 255.255.0.0, use CIDR Expressed as /16
• C Class network default subnet mask ： 255.255.255.0, use CIDR Expressed as /24

（2） Custom subnet mask

After dividing a network into subnets , Give a part of the original host number location to the subnet number , The rest is the host number of the subnet . Its form is as follows ：

After the subnet division IP Address ： network number ＋ Subnet number ＋ Subnet host number , for instance ：

• 192.168.1.100/25, Its subnet mask indicates ：255.255.255.128, It means that you will 192.168.1.0 The highest host bit of this network segment 1 The bits are divided into subnets .

gateway

Computer gateway （Gateway） Is the exit to other network segments , That is, the router interface IP Address . The router interface uses IP The address can be any address in this network segment , However, the first available address or the last available address of the network segment is usually used , This is to avoid conflicts with the host addresses in this network segment as much as possible .

In the following topology illustration example ,A And B,C And D, Can communicate directly with each other （ All belong to the same network segment , You don't have to go through the router ）, however A And C,A And D,B And C,B And D They do not belong to the same network segment , So they communicate through the local gateway , Then the router according to each other IP Address , Find the exact match in the routing table IP Direct routing of addresses , So it is forwarded from the gateway interface on the other side to realize interconnection .

The default gateway

The default gateway address is set to all except the last host bit 1 Later obtained .

Let's say there's a ip The address is 193.6.7.0/24, According to the network prefix 24 Before we know 24 Bit is network bit , after 8 Bit is host bit , Therefore, the default gateway address of the network is IP Address 193.6.7.11111110（ The bold part is the host bit ）, Convert to decimal number, that is 193.6.7.254/24.

A host can have multiple gateways . The default gateway means that if a host cannot find an available gateway , Send the packet to the default gateway , This gateway handles packets . Now the gateway used by the host , It generally refers to the default gateway . The default gateway of a computer can't be specified casually , Must specify... Correctly , Otherwise, a computer will send packets to a host that is not a gateway , Therefore, it is impossible to communicate with the host of other networks . The default gateway can be set manually or automatically .

（1） Manual settings
The manual setting is suitable for a small number of computers 、TCP/IP When the parameters are basically the same , For example, there are only a few to a dozen computers . Because this method needs to be set up on every computer connected to the network “ The default gateway ”, It's very hard , Once the default gateway has to be modified due to migration and other reasons IP Address , Will bring a lot of trouble to the network management , So... Is not recommended .

stay Windows 9x in , Set the default gateway in “ Online neighbors ” Right click on , Click... In the pop-up menu “ attribute ”, In the network properties dialog box, select “TCP/IP agreement ”, Click on “ attribute ”, stay “ The default gateway ” Tab, fill in the name of the new default gateway IP The address will do .

Here's the thing to watch out for ： The default gateway must be in the network segment of the computer itself IP Address , You can't fill in IP Address .

（2） Automatic setting

Automatic setting is to use DHCP（Dynamic Host Configuration Protocol, Dynamic Host Configuration Protocol ） Server to automatically assign computers in the network IP Address 、 Subnet mask and default gateway . The advantage of this is that once the default gateway of the network changes , Just change DHCP The default gateway settings in the server , Then all the computers in the network get the new default gateway IP Address . This method is suitable for large scale networks 、TCP/IP Parameters may change in the network . Another way to get the gateway automatically is by installing the proxy server software （ Such as MS Proxy） The client program to automatically get , Its principle and method and DHCP There are similarities . Limited space , Let's not go into details .