Networker Interview

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IP Header Interview Questions and Answers

Which is the importance of the identification field in the IP packet?
This is used to identify each fragmented packet so that destination device can rearrange the whole communication in order.

Which device can reassemble the packet?
This is done only by the ultimate destination of the IP message.

What is IP datagram?
IP datagram can be used to describe a portion of IP data. Each IP datagram has a set of fields arranged in order. IP datagram has following fields Version, Header length, Type of service, Total length, checksum, flag, protocol, Time to live, Identification, Source IP Address and Destination Ip Address, Padding, Options and Payload.

What is MTU (Maximum Transmission Unit)?
The maximum transmission unit (MTU) of an interface tells Cisco IOS the largest IP packet that can be forwarded out on that interface.

What is Fragmentation?
Fragmentation is a process of breaking the IP packets into smaller pieces (fragments). Fragmentation is required when the datagram is larger than the MTU. Each fragment then becomes a datagram in itself and transmitted independently from source. These datagrams are reassembled by the destination.

How the packet is reassembled?
1. When a host receives an IP fragment, it stores this fragment in a reassembly buffer based on its fragment offset field.
2. Once all the fragments of the original IP datagram are received, the datagram is processed.
3. On receiving the first fragment, a reassembly timer is started.
4. If this reassembly timer expires before all the fragments are received then the datagram is discarded.

What is the importance of DF, MF flag?
Don’t fragment bit
If DF bit is set, fragmentation is not allowed.
When a router needs to forward a packet larger than the outgoing interface’s MTU, the router either fragment the packet or discards it. If the IP header’s Do Not Fragment (DF) bit is set, means fragmentation is not allowed and the router discards the packet. If the DF bit is not set, means fragmentation is allowed and the router can perform Layer 3 fragmentation on the packet.

More fragments bit
If MF Bit is set to 1 means more fragments are coming. If it is set to 0 means this is the last Fragment.
All fragments that belong to an IP datagram will have more fragments bit set except for the final fragment. The final fragment does not have the more fragment bit set indicating that this is the last fragment. This is how the end hosts come to know that it has collected all the fragments of the IP datagram.

What is the purpose of a fragment offset?
It is used to define the size of each fragmented packet.

What is the importance of TTL value?
It defines how long a packet can travel in the network. It is the number of hops that the IP datagram will go through before being discarded. At every hop, the TTL value is decremented by 1. When this field becomes zero, the datagram is discarded. This behaviour helps prevent routing loops. The typical value for a TTL field is 32 or 64.

What does the protocol field determine in the IP packet?
The Protocol field is an 8-bit field that identifies the next level protocol. It indicates to which upper-layer protocol this datagram should be delivered.
Example - TCP, UDP.

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