Port Numbers

Port Numbers are very important in all IP communications. Ports are a numerical value from 0 to 65,535. Since any IP device may have multiple simultaneous communications, or multiple devices behind NAT may be accessing resources simultaneously, port numbers help to determine which application needs to receive which data. Port numbers are divided into three categories: well-known, registered, and Dynamic (or Private) ports. Well-known ports are ports 0-1,023. They are registered with the IANA and are used for specific applications. Ports 1,024-49,151 are registered ports, and are also registered with the IANA. Ports 49,152-65,535 are dynamic ports that can be used for any application. There are two ports identified in a packet, the source port number and the destination port number. Two or more IP-enabled devices can use the same port number, as the IP address for the packet will remain unique. As a combination of the two, we have a unique method of identifying which application should receive data based on the IP address and the port number.