Yu Cheng (Jade)
ICS 351
Prelab Report 3
September 23
[Question 1]
What is te IOS command to change the MTU (Maximum Transmission Unit) for an
interface on a Sisco router?
First change the specified interface using command "interface ethernet <0-0>" in
the Config Mode. Then, change the MTU value using command "ip mtu <value>" in
the Configif Mode.
[Question 2]
How does a router determine wheter datagrams to a particular host can be
directly delivered through one of its interfaces?
The datagrams come in the form of packets. Packets have IP headers. A router
tries to match up the IP headers of the packets passing by with its routing
table entities. If they don't match with any of the entities, the packets will
not stay in the network. If they do match with one of the entities, by looking
at the routing table, the router gets to know what interface to use to finish
this delivery, and then send the packet through that interface.
[Question 3]
Which systems generate ICMP route redirect messages--routers, hosts, or both?
I think ICMP route redirect messages are generated by routers.
If there is a more direct path to send packets from the source host to the
destination host, the router that's sitting in between would send out a ICMP
route redirect message to the source host. This message would inform the source
host to route through another router, which provides a more direct path to that
certain destination host. This is done by the router at the same time when it
correctly sends out the already received packets to the next hop, and from then
on, the packets from this source host to this destination host would not go
through this router any more.
[Question 4]
What is the default maximum TTL value used by traceroute when sending UDP data-
grams?
The default maximum TTL value sending UDP datagrams is 30. If the target is
within this distance, the traceroute would keep trying till finds the target.
If the target is father than 30 hops away, then this value will need to be
increased.
[Question 5]
Describe the role of a default gateway in a routing table.
The default gateway is chosen when an IP address doesn't belong to any other
entities in the routing table. The routing table decides whether the packets
should stay or leave the network. If the packets' IP header doesn't match
any of the other entries of the routing table, then it matches up with the
default gateway, then the packets will be sent out and leave the network
[Question 6]
What is the network prefix of IP address 192.110.50.3/24
The network prefix of this IP address is /24.
[Question 7]
Explain the differece between a network IP address and a network prefix.
The network IP prefix identifies the number of sigificant bits used to idetify
a network. For example, IP address 192.110.50.3/24, means the first 24 bits are
used to represent the network and the remaining 8 bits are used to identify
host.
The network IP address is the IP address network part. Network prefix is the
bit size of the network IP address.
[Question 8]
An orgaization has been assigned the network number 140.25.0.0/16 and it needs
to create networks that support up to 60 hosts on each IP network. What is the
maximum number of networks that can be set up? Expain your answer.
32 - 16 = 16
ceiling [log_2 60] = 6
16 -6 = 10
2^10 = 1024
ceiling[log_2 60] = 6 so 6 bits are required for 60 hosts, leaving 10 bits
available for the subnets. 10 bits is enough for 1024 subnets since
2^10 = 1024.
The organizaton can set up a maximum number of 1024 networks with each network
holds up to 60 hosts on the same network.
