Alcatel AST570, User Manual

Page: 189 / 300

Introduction
Features
Configuring an IP
routing table
14 Networking Services - IP
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3EC 17766 AAAA TCZZA Ed. 04
14.5.1 The AST570 IP Router
Because the AST570 can act as an IP router, it has the ability to
access machines in other networks than its own. This can
be achieved by adding specific routes to its IP routing table.
This subsection provides some general information on the
AST570 IP router functionality.
IP routing:
Is a standard and a wellknown principle, mainly due to the
widespread Internet use
Has broad application support, as it is implemented in most,
if not all Operating Systems (Windows, Unix, Mac OS, ...).
The routes in an ordinary routing table or Forwarding Information
Base (FIB) include, among others, destination IP addresses, subnet
masks and gateways.
When an IP packet arrives at the router, the router examines
the destination IP address. The router looks up the most specific
match in the routing table for that destination address. Finding the
most specific match equals finding the longest subnet mask for
that IP address.
For example, the subnet mask 255.255.255.0 is more specific
than 255.255.0.0 because the network part in the first case is
longer (and thus more specific) than the network part in the
second case.
Once the most specific match is found, the router forwards the IP
packet to the gateway associated with that match.