I know the IP header length is a minimum of 20 bytes and a maximum of 60 bytes. But why? Why is the IP header length limited to 60 bytes? Is there anybody here that can explain it to me? Thank you.
Maximum IPv4 header size
ipv4protocol-theory
ipv4protocol-theory
I know the IP header length is a minimum of 20 bytes and a maximum of 60 bytes. But why? Why is the IP header length limited to 60 bytes? Is there anybody here that can explain it to me? Thank you.
Best Answer
The Internet Protocol (IP) is defined in RFC 791. The RFC specifies the format of the IP header. In the header there is the IHL (Internet Header Length) field which is 4 bits long and specifies the header length in 32 bit words. The IHL field can hold values from 0 (Binary 0000) to 15 (Binary 1111).
So the longest Internet Header (IP header) size can be 15*32 Bits = 480 Bits = 60 Bytes. This is why the header has a maximum size of 60 Bytes.
The shortest header size is 20 bytes, where the IHL field has the value 5 (0101). This is because all the required fields in the header need 20 Bytes of space. So while in theory you could set the IHL to a value < 5 this would always be an incorrect value and thus an invalid packet header.