ABSTRACT
Figure 1 shows the fields of a TCP segment, the part of an IP packet that follows the IP header information. The first 16 bits identify the source port, and the second 16 bits identify the destination port. Port numbers provide a way for IP hosts to multiplex numerous types of concurrent connections at a single IP address. The combination of a 32-bit IP address and a 16-bit port address identifies a socket in most modern operating systems. The combination of a source socket and a destination socket defines a TCP connection. There are 216 or 65,536 possible ports. The lowest 1,024 ports are called well-known ports; these are set aside by default for particular Application layer protocols. For example, HTTP uses port 80 by default, while POP3 uses port 110. Other applications can use the higher port numbers.
