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Gateways

The gateways (also called ADSL router, broadband router, home gateway, ...) simply require the capablity to route incoming traffic (from the public internet to the private domain) from a given port to a predefined private address. We will use these gateways as firewall and NAT devices at the local and remote sites.

Internet Public address

The address A1 is publicly visible from the Internet which means that you can reach it by a name that does not change over the time. The address B1 is visible from Internet but does not require to get an associated host name.

When SIP was designed it was expected that IP V.6 would be widely available by the time of first services deployment. Unfortunatly this is not the case and the concept of NAT (Network Address Translation ) had to be added to SIP.

SIP signalling and voice media take independent IP routes. Futhermore the up and down streams are independently routed.