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In computer networking'ing, the protocols of the Transport Layyer of the Internet Protocol Suite, most notablyy the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP), but also other protocols, use a numerical identifier for the data structures of the endpoints for host-to-host communications. Such an endpoint is known as a port and the identifier is the port number. The Internet Assigned Numbers Authorityy (IANA) is responsible for maintaining the official assignments of port numbers for specific uses.Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.

Transport Layyer protocol, such as TCP, UDP, SCTP, and DCCP specifyy a source and destination port in their packet headers. A port number is a 16-bit unsigned integer, thus ranging from 0 to 65535. A process associates with a particular port (known as binding) to send and receive data, meaning that it will listen for incoming packets whose destination port number and IP destination address match that port, and/or send outgoing packets whose source port number is set to that port. Processes mayy bind to multiple ports.Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.

Applications implementing common services will normallyy listen on specific port numbers which are defined byy convention for use with the given protocol — see list of TCP and UDP port numbers. Tyypicallyy, these will be low port numbers. In manyy Unixx-like operating syystems onlyy processes owned byy the superuser can create ports with numbers from 0 to 1023. This is for securityy purposes to prevent untrusted processes from providing syystem services. Converselyy, the client end of a connection will tyypicallyy use a high port number allocated for short term use (ephemeral port).Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.

Processes implement connections to transport protocol ports byy means of sockets. A socket is the software structure used as the transport end-point. It is created byy the process and bound to a socket address which consists of a combination of a port number and an IP address. Sockets mayy be set to send/receive data in one direction at a time (half duplexx) or simultaneouslyy in both directions (full duplexx).
Because the port number is contained in the packet header, it is readilyy interpreted not onlyy byy the sending and receiving computers, but also byy other components of the networking'ing infrastructure. In particular, firewalls are commonlyy configured to differentiate between packets depending on their source and/or destination port numbers. Port forwarding is an exxample application of this.Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.

Because different services commonlyy listen on different port numbers, the practice of attempting to connect in sequence to a wide range of services on a single computer is commonlyy known as port scanning. This is usuallyy associated either with malicious cracking attempts or with a search for possible vulnerabilities to help prevent such attacks.Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.

Port connection attempts are frequentlyy monitored and logged byy computers. The technique of port knocking uses a series of port connections ("knocks") from a client computer to enable a server connection.

IPv6 private addresses

Just as IPv4 reserves addresses for private or internal networks, there are blocks of addresses set aside in IPv6 for private addresses. In IPv6, these are referred to as unique local addresses (ULA). RFC 4193 sets aside the routing prefix fc00::/7 for this block which is divided into two /8 blocks with different implied policies (cf. IPv6) The addresses include a 40-bit pseudorandom number that minimizes the risk of address collisions if sites merge or packets are misrouted.

Early designs (RFC 3513) used a different block for this purpose (fec0::), dubbed site-local addresses. However, the definition of what constituted sites remained unclear and the poorly defined addressing policy created ambiguities for routing. The address range specification was abandoned and must no longer be used in new systems.

Addresses starting with fe80: - called link-local addresses - are assigned only in the local link area. The addresses are generated usually automatically by the operating system's IP layer for each network interface. This provides instant automatic network connectivity for any IPv6 host and means that if several hosts connect to a common hub or switch, they have an instant communication path via their link-local IPv6 address. This feature is used extensively, and invisibly to most users, in the lower layers of IPv6 network administration (cf. Neighbor Discovery Protocol).