Consider an IP packet with a data length of 4400 bytes. TCP header is ...
IP Packet Fragmentation
IP packet fragmentation is a process in which a large IP packet is divided into smaller fragments to be transmitted over a network with a smaller Maximum Transmission Unit (MTU). Each fragment contains a portion of the original packet along with additional IP header information.
Given Information:
- Data length of the IP packet: 4400 bytes
- TCP header size: 40 bytes
- IPv4 header size: 20 bytes
- MTU of the IPv4 router: 900 bytes
- IP header size for outgoing fragments: 20 bytes
- Fragment offset value of the first fragment: 100
Explanation:
To determine the fragmentation offset value of the penultimate fragment, we need to calculate the size of each fragment and the number of fragments required to transmit the entire IP packet.
Step 1: Calculate the size of each fragment
The maximum size of each fragment is determined by the MTU of the IPv4 router. Subtracting the IP header size for outgoing fragments (20 bytes) from the MTU gives us the payload size for each fragment:
Fragment payload size = MTU - IP header size for outgoing fragments
= 900 - 20
= 880 bytes
Step 2: Calculate the number of fragments
To determine the number of fragments required, we divide the total IP packet size (including TCP and IPv4 headers) by the fragment payload size:
Number of fragments = ceil((Data length + TCP header size + IPv4 header size) / Fragment payload size)
= ceil((4400 + 40 + 20) / 880)
= ceil(4460 / 880)
= ceil(5.068)
= 6
Step 3: Calculate the fragmentation offset value of the penultimate fragment
The fragmentation offset value of each fragment is calculated by multiplying the fragment index (starting from 0) by the fragment payload size. The fragmentation offset value of the penultimate fragment (5th fragment) can be calculated as:
Fragmentation offset value of penultimate fragment = (Fragment index) * (Fragment payload size)
= (5) * (880)
= 4400
However, since the fragmentation offset value of the first fragment is given as 100, we need to subtract it from the calculated value to get the relative fragmentation offset value:
Fragmentation offset value of penultimate fragment = 4400 - 100
= 430
Therefore, the fragmentation offset value of the penultimate fragment is 430.
Consider an IP packet with a data length of 4400 bytes. TCP header is ...

Data = 4400 Byte
Router MTU = 900 byte
Data + header = 900
Data = 900 – 20
∴ Data = 880
Number of fragments = 4400/880 = 5
Since initial fragment offset is 100.
Fragment offset of 2nd fragment = 100 + 880/8 = 210
Fragment offset of 3rd fragment = 210 + 880/8 = 320
Fragment offset of 4th fragment = 320 + 880/8 = 430
Fragment offset of 5th fragment = 430 + 880/8 = 540
penultimate fragment = 2nd last fragment = 430
Shortcuts:
penultimate fragment = Initial fragment offset + 110 × (n - 2)
where n is number of fragments