From: Rajiv (electroheart143@yahoo.com)
Date: Mon Jan 29 2007 - 18:58:50 ART
Hello Guys
I know this is kind of silly question that I am gonne ask.I am currently reading TCP/IP by Douglas E. Comer.In the IP header format,a field named Total Length it is written that it is
216 -1 which means 65,535 octets or bytes,I know that field is 16 bits long,but why it has been put to 2 to the power 16?
Also he has written one line(page 43,Fifth Edition of TCP/IP by Comer):-
"Because IP address encode both a network and a host on that network,an address does not specify an individual computer,but a connection to a network"
But I personally think,in some cases(correct me if I am wrong) that IP address does specify a individual computers,like in case if I use Public IP address on a server,then in that case that IP address WOULD specify an individual computer also,I think I am right or maybe I misunderstood what he wanna say?
Also I really not getting one more thing,like the Header Lenght field is 4 bit long,but its written that is contains 32 bit words,I mean I am not getting in this thing is that how can 4 bit field can contain "32" BIT WORD? Also one more thing I am confused is that it is written:-
"The minimum length of IP header is 20 octets/bytes and maximum is 60 octets"
Now how is this calculated,when I added all the fields in header like Version,TTL,Protocols etc... then it isnt coming out to be ard 20 bytes/octets or I am again misunderstanding this thing,I really have no idea where is this 20 ocet/60 octet thing came,Right nwo I am really looking at the diagram of IP Header:-
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In this diagram at the TOP i can see that it is 32 bit TOTAL,is this total length of header?,well if it is then what is the 20 octet thing coming from or then how can alone Total Length field can alone have 32 bit word(or is tihs 32 bit some kind of naming bit,really dont knw what it is,my brain is really tosted now,searched every book and RFC and cant find my answers,that's y posting them at last at GS).Also where that 2 came or how "216 -1" thing is derived in that Total Length Field.Help me!
Rajiv
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This archive was generated by hypermail 2.1.4 : Thu Feb 08 2007 - 23:46:57 ART