IP packet header length wrt option field

[rtwwpad]

Hi

I have a question on the length of the IP packet headers. Reading most documentation it seems to suggest that the lenght of an IP packet header should be 160-192 bits dependent on whether or not the options field is utilised. However if this is the case I am confused as to where the information for the loose/strict source routing is stored. As the option field can only be 32 bits in size, to keep things inside 192 bits I can't see how I can specify two hops a packet must go through.

Is it just that the information has been misrepresented or I am misinterpreting it. I am referring to Jeff Doyles book on Routing Vol1 pg 33 and other internet based resources. I have checked the RFC but I cannot find its reference to max header length.

Could someone please explain to me where this information (the routing hops) is kept.

Kind regards

Phil Thomas

[Harold Ritter]

The IP header length is 4 bit field therefore the maximum length would be 15 (F in hexa) * 4bytes = 60 bytes. I don't know where that 192 bits (24 bytes) comes from but it is definitely wrong.

Hope this helps,

[Andrew Sparkes]

I know some time has passed since this post was first commented on, but I can understand the confusion with the IP header. The header it's self has 14 fields that equate to 192 bits if you add each portion of the IP header together. However, with all binary numbers its the octets that need to be counted and not the individual bits. Therefore, there are 60 possible octets in the IP header, but without the options field this drops to 20.

So at a minimum the IP header will be 20 Octets or 160 bits long

Maximum 60 octets or 480 bits long.

[paolo bevilacqua]

Andrew Sparkes wrote:
I know some time has passed since this post was first commented on, but I can understand the confusion with the IP header. The header it's self has 14 fields that equate to 192 bits if you add each portion of the IP header together. However, with all binary numbers its the octets that need to be counted and not the individual bits. Therefore, there are 60 possible octets in the IP header, but without the options field this drops to 20.
So at a minimum the IP header will be 20 Octets or 192 bits long
Maximum 60 octets or 440 bits long.

Not sure what are you on about, but you bit math is all wrong. Harold's answer above is correct.

1 Octet = 1 Byte = 8 bits.

20 Octets = 20 x 8 = 160 bits, not 192.

60 Octets = 60 x 8 = 480 bits, not 440.

[Andrew Sparkes]

Correct, Fat finger issue and not proof reading the post which was a massive mistake.

[paolo bevilacqua]

No problem. Just FYI, there is an option to delete own postings and replies too, that I find convenient at times.