Understanding Source NAT Pools with PAT

Using the source pool with Port Address Translation (PAT), JUNOS Software translates both the source IP address and the port number of the packets. When PAT is used, multiple hosts can share the same IP address.

JUNOS Software maintains a list of assigned port numbers to distinguish what session belongs to which host. When PAT is enabled, up to 64,500 hosts can share a single IP address. Each source pool can contain multiple IP addresses, multiple IP address ranges, or both. For a source pool with PAT, JUNOS Software may assign different addresses to a single host for different concurrent sessions, unless the source pool or JUNOS Software has the persistent address feature enabled.

For interface source pool and source pool with PAT, range (1024, 65535) is available for port number mapping per IP address. Within range (1024, 63487) one port is allocated at a time. In range (63488, 65535), two ports are allocated at a time for RTP/RTCP applications such as SIP, H.323, and RTSP.

When a host initiates several sessions that match a policy that requires network address translation and is assigned an address from a source pool that has PAT enabled, the device assigns a different source IP address for each session. Such random address assignment can be problematic for services that create multiple sessions that require the same source IP address for each session. For example, it is important to have the same IP address for multiple sessions when using the AOL Instant Message (AIM) client.

To ensure that the router assigns the same IP address from a source pool to a host for multiple concurrent sessions, you can enable a persistent IP address per router.

Understanding Source NAT Pools with PAT

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