One 、 Concept

Subnet mask （subnet mask） Also called netmask 、 Address mask , It is used to indicate One IP Which bits of the address identify the subnet where the host is located , And which bits identify the host number mask . The subnet mask cannot exist alone , It has to be combined with IP Address used together . The subnet mask has only one function , It's about putting some IP The address is divided into network address and host address .

If a computer IP The address is configured to 172.16.122.204, The subnet mask is 255.255.0.0. Subnet mask sum IP Address and operation , The network segment of the computer is 172.16.0.0.

Two 、 Illustrations demonstrate

192.168. 80. 7   # IP Address 
255.255.255. 0   #  Subnet mask 
192.168. 80. 1   #  gateway 
 

192.168. 90. 9   # IP Address 

As shown above ,IP Address 192.168. 80. 7 The subnet mask of is 255.255.255. 0, So the IP The network segment of is 192.168. 80.0 Network segment . from IP Address 192.168. 80. 7 The purpose of the sent packet IP The address is 192.168. 90. 9 Words ,192.168. 80. 7 According to their own subnet mask 255.255.255. 0, And purpose IP Address 192.168. 90. 9 Do with the operation , Judge the destination address network segment as 192.168. 90.0 Network segment .192.168. 80.0 and 192.168. 90.0 Does not belong to the same network segment , So from IP Address 192.168. 80. 7 Data sent , Send it to your gateway 192.168. 80. 1 , Then the gateway sends it to the router and forwards it .

192.168. 80. 7   # IP Address 
255.255.  0. 0   #  Subnet mask 
192.168. 80. 1   #  gateway 
 

192.168. 90. 9   # IP Address 

As shown above ,IP Address 192.168. 80. 7 The subnet mask of is 255.255.0. 0, So the IP The network segment of is 192.168. 0.0 Network segment . from IP Address 192.168. 80. 7 The purpose of the sent packet IP The address is 192.168. 90. 9 Words ,192.168. 80. 7 According to their own subnet mask 255.255.0. 0, And purpose IP Address 192.168. 90. 9 Do with the operation , Judge the destination address network segment as 192.168. 0.0 Network segment .192.168. 0.0 and 192.168. 0.0 Belong to the same network segment , So from IP Address 192.168. 80. 7 Data sent , Will be forwarded in the local network , It will not be sent to the router .