Next Available Port

In this article we will discuss Next Available Port, will make brief discussion on Next Available Port, In last article we discuss about Port Address Translation (PAT).

The client port numbers, 1331 and 1555, did not change at the NAT-enabled router. This is not a very likely scenario, because there is a good chance that these port numbers may have already been attached to other active sessions.

PAT attempts to preserve the original source port. However, if the original source port is already used, PAT assigns the first available port number starting from the beginning of the appropriate port group 0–511, 512–1,023, or 1,024–65,535.

When there are no more ports available and there is more than one external address in the address pool, PAT moves to the next address to try to allocate the original source port. This process continues until there are no more available ports or external IPv4 addresses.

In the animation, the hosts have chosen the same port number 1444. This is acceptable for the inside address, because the hosts have unique private IPv4 addresses. However, at the NAT router, the port numbers must be changed; otherwise, packets from two different hosts would exit R2 with the same source address. This example assumes that the first 420 ports in the range 1,024-65,535 are already in use, so the next available port number, 1445, is used.

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