User guide

IP support on AT-iMG Models IP
4-3
iMG/RG Software Reference Manual (IPNetwork Functions)
The maximum number of IP interfaces is set to 16, which means that there are up to 16 IP interfaces
internally numbered one to 16. Since one interface is reserved for use as a loopback interface, this means up to
15 IP interfaces can be added by the user
When a packet arrives on an IP interface, the IP stack determines what to do with the packet. There are two
options:
• Receive the packet locally;
• Forward the packet to another interface
4.1.3.2 IP stack and incoming packets
When a packet arrives on an IP interface, the IP stack determines whether:
• The packet should be received locally
• The packet should be forwarded to another interface
4.1.3.3 Locally received packets
A packet will be received locally if:
• The destination address of the packet matches any of the IP stack interface addresses (real or virtual inter-
face, primary or secondary addresses)
• The packet is a broadcast
• The packet is a multicast to a group that the IP stack belongs to
• The packet has the Router Alert option set
The packet is either processed internally within the IP stack (for example, ICMP or IGMP control messages), or
passed up to an application via the appropriate protocol processing (for example, TCP or UDP data).
For a local application to successfully send a packet back to another host, the IP stack must be able to find a
suitable route to that host.
4.1.3.4 Forwarding packets
If the IP stack determines that a packet should not be received locally, it will try to forward the packet. The
packet will be forwarded if:
• The destination of the packet can be reached directly via any of the IP stack’s interfaces
• A route has been added, either manually or by a routing protocol, specifying a suitable gateway via which
that destination may be reached
Several address tests are applied before forwarding a packet, for example to prevent broadcast packets from
being forwarded. For more information about these tests, see RFC1122: Requirements for Internet - Hosts.