Questions and Answers on routing fundamentals

1. Routing Fundamentals
Question
Routers work at the network layer of the OSI model. Does this imply that they do not have the capacity
to check for information inside frames?
Answer
Routers look for information inside the IP packet header for routing the packets. But the packets itself
would be encapsulated inside frames like ethernet, PPP etc. For example, if the IP packet is
encapsulated inside an ethernet frame, the router would first need to check the destination mac-
address inside the frame and verify if it is intended for itself and only then be able to process the IP
packet header, which would be the data inside the packet.
Question
How do routers populate routing entry for a directly connected network ?
Answer
A directly connected network refers to the network address corresponding to the IP address and subnet
mask configured on the specific network interface card on the router. The IP address and subnet mask
would be “And” ed to derive the network address.
Question
How do routers forward IP packets?
Answer
When a router receives an IP packet, it would check the destination IP address in the packet. The router
would then performing an “And “ operation with the subnet mask of the different route table entries
with the destination IP address and verify if the corresponding network address in the routing table
matches. On a match, the packet would be forwarded to the interface to which it the route entry is
associated with.
Question
What are the parameters that are provided in a static route?
Answer
A static entry would typically contain the network address, network mask and the default gateway. Let’s
table an example where a router has two directly connected networks. The hosts within the router,
needs to access a network which is not a directly connected network. The network is one hop away from
the router and is reachable via the default gateway of the router. So the network address value would

2. be the one which the hosts would want to reach via the router. This network address along with the
network mask and the gateway address through which the network can be reached is configured as a
static route.
Question
How does a PC know as to whether the destination IP address is on the same or different network?
Answer
Assume that a PC is configured with an IP address 192.168.2.1 and subnet mask 255.255.255.0. If the
user on the PC issues a ping to the IP address 192.168.1.1, the PC would check in its routing table for
matching network address and decide if it is on the same or different network.
Question
An ADSL router is configured for internet connection. A switch is connected to the router to which the
hosts are connected. What type of route entry should be configured on the router for the hosts to
access the internet?
Answer
A default route entry should be configured on the router. IP addresses which are internet based, cannot
be configured explicitly on the routers since they would not be known beforehand. Examples are IP
addresses of websites which the user tries to access. The default route once configured would take care
of destination networks which the router is unaware.