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Unformatted text preview: efault entry. (The default entry is normally specified in the routing
table as a network entry, with a network ID of 0.)
A matching host address is always used before a matching network address.
The routing done by IP, when it searches the routing table and decides which interface to
send a packet out, is a routing mechanism. This differs from a routing policy, which is a set
of rules that decides which routes go into the routing table. IP performs the routing
mechanism while a routing daemon normally provides the routing policy. file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/ip_rout.htm (2 of 17) [12/09/2001 14.46.50] Chapter 9. IP Routing Simple Routing Table
Let's start by looking at some typical host routing tables. On the host svr4 we execute the
netstat command with the -r option to list the routing table and the -n option, which
prints IP addresses in numeric format, rather than as names. (We do this because some of
the entries in the routing table are for networks, not hosts. Without the -n option, the
netstat command searches the file /etc/networks for the network names. This
confuses the discussion by adding another set of names-network names in addition to
svr4 % netstat rn
188.8.131.52 U Refcnt
emd0 The first line says for destination 184.108.40.206 (host slip) the gateway (router) to send
the packet to is 220.127.116.11 (bsdi). This is what we expect, since the host slip is
connected to bsdi with a SLIP link, and bsdi is on the same Ethernet as this host. There
are five different flags that can be printed for a given route.
U The route is up.
G The route is to a gateway (router). If this flag is not set, the destination is directly
H The route is to a host, that is, the destination is a complete host address. If this flag is not
set, the route is...
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