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IP multicasting whenever possible, router discovery can work using broadcast messages also. 9.7 Summary
The operation of IP routing is fundamental to a system running TCP/IP, be it a host or
router. The routing table entries are simple: up to 5 flag bits, a destination IP address (host, file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/ip_rout.htm (16 of 17) [12/09/2001 14.46.51] Chapter 9. IP Routing network, or default), a next-hop router IP address (for an indirect route) or a local interface
IP address (for a direct route), and a pointer to a local interface to use. Host entries have
priority over network entries, which have priority over default entries.
A search of this routing table is made for every IP datagram that the system generates or
forwards, and can be updated by either a routing daemon or ICMP redirects. By default a
system should never forward a datagram unless it has specifically been configured to do so.
Static routes can be entered using the route command, and the newer ICMP router
discovery messages can be used to initialize and dynamically update default entries. Hosts
can start with a simple routing table that is updated dynamically by ICMP redirects from its
Our discussion in this chapter has focused on how a single system uses its routing table. In
the next chapter we examine how routers exchange routing information with each other.
9.1 Why do you think both types of ICMP redirects-network and host-exist?
9.2 In the routing table for svr4 shown at the beginning of Section 9.2, is a specific route
to the host slip (188.8.131.52) necessary? What would change if this entry were
removed from the routing table?
9.3 Consider a cable with both 4.2BSD hosts and 4.3BSD hosts. Assume the network ID is
140.1. The 4.2BSD hosts only recognize a host ID of all zero bits as the broadcast address
(184.108.40.206), while the 4.3BSD hosts normally send a broadcast using a host ID of all one
bits (220.127.116.11). Also, the 4.2BS...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12