GATE Overflow Test Series | Computer Networks | Test 1 | Question: 5

Question

An IP datagram of size $1200\;\text{bytes}$ arrives at a router. The router has to forward this packet on a link whose MTU (maximum transmission unit) is $120\;\text{bytes}$. Assume that the size of the IP header is $20\;\text{bytes}$. The number of fragments that the IP datagram will be divided into for transmission is __________

Answer 1

We have to sent $1200\;\text{bytes}$ out of which $20$ bytes is header. So, a tual data to be transmitted $=1180$ bytes.    

Each fragment can have $100\;\text{bytes}$ data and $20\;\text{bytes}$ header as MTU is $100+20=120\;\text{bytes}$. But, the payload must be a multiple of $8$ as the offset field uses multiple of $8$ as it's unit. This means the router can only sent $96$ bytes of data in a fragment.
    
So, to send $1200\; \text{bytes}$ the router needs $\left\lceil \dfrac{1200-20}{96}\right \rceil=13$ fragments.