Exercise
1. Consider the following static routing table found on a router.
Destination Gateway
Genmark
Flags Metric Ref Use
192.168.5.20
192.168.1.81
192.168.10.0
192.168.18.0
192.168.64.0
192.168.128.0
127.0.0.0
0.0.0.0
255.255.255.255
255.255.255.255
255.255.255.0
255.255.254.0
255.255.192.0
255.255.192.0
255.0.0.0
0.0.0.0
UGH
UGH
U
U
UG
UG
U
UG
192.168.10.7
192.168.10.5
0.0.0.0
0.0.0.0
192.168.10.5
192.168.10.7
0.0.0.0
192.168.10.20
1
1
0
0
1
1
0
1
0
0
0
0
0
0
0
0
Iface
180 eth1
187 eth1
63311 eth1
753430 eth0
47543 eth1
89011 eth1
564 lo
183436 eth1
a. Interpret the flags associated with each route - what specific information is conveyed by the set
of flags associated with each route?
b. Draw a rough diagram of the network in which this router is sitting.
c. Can you tell the IP addresses associated with the eth0 and eth1 interfaces on this router?
d. What are the network and broadcast addresses of the two networks that this router's interfaces
are attached to?
e. Is it always true that the Gateway column in the routing table always has a host IP or 0.0.0.0 ?
2. Explain what the flags in the routing table above signifies. More generally,
a. If a routing table entry has only the U flag, what does that mean (remember that routing table
flags are statements about the destination)?
b. If a routing table entry has UG flags, what does that say about the destination?
c. If a routing table entry has UGH flags, what does that say about the destination?
d. If a routing table entry has UH flags, what does that say about the destination?
3. Construct routing table entries (only Destination, Gateway, Genmask, Flags and Iface)
for the router R1 in the network shown below. (Note that x.y.z.w/32 means it is a host IP, not a
network address. On the other hand, it is not obvious (or is it?) if 192.168.5.128/25 is a host IP
or a network address – you will have to decide that)
192.168.16.0/21
192.168.10.0/24
192.168.10.11
R2
192.168.10.101
eth0
192.168.10.12
192.168.5.128/25
192.168.16.100
R1
192.168.100.192/26
192.168.16.120
192.168.16.101
R3
eth1
192.168.17.101
192.168.10.14
R4
0.0.0.0/0 default, Internet
192.168.254.135/32