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Answers to the Chapter 5 "Do I Know This Already?" Quiz 727
subnets later. You decide to start with the lowest numerical values for the subnet number
you will use. Assume that point-to-point serial links will be attached to this router and that
RIP is the routing protocol.
router rip
network 8.0.0.0
interface ethernet 0
ip address 8.0.1.1 255.255.255.0
interface serial 0
ip address 8.0.2.1 255.255.255.0
interface serial 1
ip address 8.0.3.1 255.255.255.0
The zero subnet was not used in this solution. If desired, the ip subnet-zero global
command could have been used, enabling subnet 8.0.0.0 as well as the subnets 8.0.1.0,
8.0.2.0, and 8.0.3.0 to be used in the configuration.
10
Describe the question and possible responses in setup mode when a router wants to know
the mask used on an interface. How can the router derive the correct mask from the
information supplied by the user?
When using versions of the IOS before version 12.0, the question asks for the number of
subnet bits. The router creates a subnet mask with x more binary 1s than the default mask
for the class of network of which the interface's IP address is a member. (x is the number
in the response.) "Number of subnet bits" from the setup question uses the definition that
there are three parts to an address--network, subnet, and host. The size of the network
field is based on the class of address; the interface's address was typed in response to an
earlier setup question. The mask simply has binary 1s in the network and subnet fields,
and binary 0s in the host field.
With version 12.0 and beyond, setup prompts for the subnet mask in canonical decimal
format--for example, 255.255.255.0.
11
Define the purpose of the trace command. What type of messages does it send, and what
type of ICMP messages does it receive?
The trace command learns the current route to a destination address. It uses IP packets
with UDP as the transport layer protocol, with TTL values beginning at 1 and then
incrementing by 1 in successive messages. The result is that intervening routers find that
the TTL is exceeded and send ICMP "TTL exceeded" messages back to the originator of
the packet, which is the router where the trace command is being executed. The source
addresses of the "TTL exceeded" packets identify each router. By sending other packets
with TTL=2, then 3, and so on, eventually the packet is received by the host. The host
returns a "port unreachable" ICMP message, which lets the trace command know that the
endpoint host has been reached.
apA.fm Page 727 Monday, March 20, 2000 5:24 PM