UNIX for Dummies Questions & Answers

netstat -rn

Routing Table: IPv4
  Destination           Gateway           Flags  Ref   Use   Interface
-------------------- -------------------- ----- ----- ------ ---------
a.b.c.d               x.x.x.1              U        1 537552 ce1
a.d.e.d               x.x.x.2              U        1 809673 ce0

Routing Algorithm
      The routing table entries are of three types:

           +  Entries for a specific host.
           +  Entries for all hosts on a specific network.
           +  Wildcard entries for any destination not matched by entries of
              the first two types.

      To select a route for forwarding an IP packet, the network facilities
      select the complete set of "matching" routing table entries from the
      routing table. A routing table entry is considered a match, if the
      result of the bit-wise AND operation between the netmask in the
      routing entry and the IP packet's destination address equals to the
      destination address in the routing entry.

      The network facilities then select from the set the routing entries
      that have the longest netmask.  The length of a netmask is defined as
      the number of contiguous 1 bits starting from the leftmost bit
      position in the 32-bit netmask field. In other words, the network
      facilities select the routing entry that specifies the narrowest range
      of IP addresses.  For example, the host route entry that has a
      destination/netmask pair of (147.253.56.1, 0xffffffff), is more
      specific than the network route entry that has a destination/netmask
      pair of (147.253.56.0, 0xfffffe00), therefore the network facilities
      select the host route entry. The default route by default has a
      destination/netmask pair of (0,0). Therefore the default route matches
      all destinations but it is also the least specific.  The default route
      will be selected only if there is not a more specific route.

      There may still be multiple routing entries remaining. In that case
      the IP packet is routed over the first entry displayed by netstat -r.
      Such multiple routes include:

           +  Two or more routes to a host via different gateways.
           +  Two or more routes to a network via different gateways.
           +  Two default routes.

      A superuser can change entries in the table by using the route command
      (see route(1M), or by information received in Internet Control Message
      Protocol (ICMP) redirect messages.