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744 Appendix A: Answers to the "Do I Know This Already?" Quizzes and Q&A Sections
65
Name the two commands typically used to create a default gateway for a router.
The ip default-gateway command and the ip route 0.0.0.0 0.0.0.0 command. Both
accomplish the goal of having the router use a known route as the default for packets that
are not matched in the routing table. The ip route 0.0.0.0 0.0.0.0 uses the fact that network
0.0.0.0 is used by the IOS to represent the default network.
66
Assume that subnets of network 10.0.0.0 are in the IP routing table in a router but that no
other network and their subnets are known, except that there is also a default route
(0.0.0.0) in the routing table. A packet destined for 192.1.1.1 arrives at the router. What
configuration command determines whether the default route will be used in this case?
If the command ip classless is configured, the packet is routed using the default route. If
no ip classless is configured, then the packet will be discarded.
67
Assume that subnets of network 10.0.0.0 are in the IP routing table in a router but that no
other network and their subnets are known, except that there is also a default route
(0.0.0.0) in the routing table. A packet destined for 10.1.1.1 arrives at the router, but
there is no known subnet of network 10 that matches this destination address. What
configuration command determines whether the default route will be used in this case?
The packet will be routed using the default route, regardless of other configuration
commands. In this particular scenario, where the Class A, B, or C network is known, there
is no match for the destination in the known subnets, and a default exists, the default must
be used.
68
What does the acronym CIDR stand for? What is the original purpose of CIDR?
Classless Interdomain Routing is the concept of grouping multiple sequential network
numbers into a single routing table entry, for the purpose of improving scalability of
Internet routers by reducing the size of the IP routing table.
69
Define the term private addressing as defined in RFC 1918.
Some hosts will never need to communicate with other hosts across the Internet. For such
hosts, assignment of IP addresses from registered networks wastes IP addresses. To
conserve IP addresses, a set of network numbers, called private addresses, has been
reserved and can be used in these cases to help conserve IP addresses for use over the
Internet.
70
Define the acronym NAT and the basics of its operation.
Network Address Translation is a mechanism for allowing hosts with private addresses or
addresses that conflict with IP addresses from a registered network to communicate with
hosts over the Internet. The basic operation involves the NAT router changing the IP
addresses in packets to and from these hosts so that the only type of IP address used in
flows through the Internet uses addresses that are legitimately registered.
apA.fm Page 744 Monday, March 20, 2000 5:24 PM