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Answers to the Chapter 5 Q&A Section 739
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.
37
In the previous question, what would be the IP subnet of the link attached to serial 0? If
another user wanted to answer the same question but did not have the enable password,
what command(s) might provide this router's addresses and subnets?
The attached subnet is 8.0.2.0, 255.255.255.0. The show interface, show ip interface,
and show ip interface brief commands would supply this information, as would show ip
route
. The show ip route command would show the actual subnet number instead of the
address of the interface.
38
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.
39
Name the three classes of unicast IP addresses and list their default masks, respectively.
How many of each type could be assigned to companies and organizations by the NIC?
Class A, B, and C, with default masks 255.0.0.0, 255.255.0.0, and 255.255.255.0,
respectively. 2
7
Class A networks are mathematically possible; 2
14
Class Bs are possible;
and 2
21
Class C networks are possible. There are two reserved network numbers in each
range.
40
Describe how TCP performs error recovery. What role do the routers play?
TCP numbers the first byte in each segment with a sequence number. The receiving host
uses the acknowledgment field in segments it sends back to acknowledge receipt of the
data. If the receiver sends an acknowledgment number that is a smaller number than the
sender expected, the sender believes that the intervening bytes were lost, so the sender
resends them. The router plays no role unless the TCP connection ends in the router--for
example, a telnet into a router. A full explanation is provided in the section, "Error
Recovery (Reliability)" in Chapter 5.
apA.fm Page 739 Monday, March 20, 2000 5:24 PM