CODEWITHSUNDEEP
network-programmingipipv4
Amit Singh Tomar
Amit Singh Tomar

Reputation: 8610

How do I calculate IP-header length?

I want to calculate IP-header length with following statement:

Header Length is a four-bit field that tells, as the name implies, the length of the IP header in 32-bit words

Now I'm getting difficulties in calculating IP header length (minimum and maximum), with four-bit field.

Upvotes: 5

Views: 33086

Answers (4)

terwxqian
terwxqian

Reputation: 659

The Internet Header Length (IHL) field is the number of 32-bit words(=4 bytes) in the IPv4 header, including any options. Because this is also a 4-bit field, the IPv4 header is limited to a maximum of fifteen 32-bit words(=60 bytes)

So if the value of IHL is 0101(5), then the IP-header's length is 5*4(bytes) = 20(bytes)

Upvotes: 0

Vikrant Singh
Vikrant Singh

Reputation: 689

minimum value of header length is 20 Bytes but we don't have sufficient bits to represent 20 so we use scaling technique, i.e. 0101 (5) will represent 4 X 5 = 20 Bytes, here scaling factor is 4.

maximum value possible with 4 bits is 15.

So maximum header length possible is 4 X 15 = 60 Bytes.

Header Length |  Header Length Field

         20 -----> 5
         24 -----> 6
         28 -----> 7
            .
            .
            .
         60 -----> 15

if header length is 22 Bytes then we use padding to make it a multiple of 4 i.e. 24 Bytes

Upvotes: 5

Poorvi
Poorvi

Reputation: 125

@Amit>Value in the HL filed is = the number of 4 bytes in the total IP header length.Means if the header length field is say 40,then calculate how many 4 bytes in 40?its 40/4= 10.So value in the HL field is 10

MINIMUM HL value

Minimum HL is 20 byte .ie no of 4bytes in 20 = 20/4=5.So minimum value in HL field is 5.

MAXIMUM HL value

HL is a 4 bit field.so the maximum vale that can be accomadated in that field is 15(1111) or you can calculate using the formulae 2^4-1=15.So max no: of 4 bytes can be 15.Hence the Max header length=15*4=60bytes.

Hope now things are clear.

Upvotes: 1

cnicutar
cnicutar

Reputation: 182619

  • The minimum length of an IPv4 header of a valid datagram is 20 bytes, when the value reads 5 and there are no options
  • Since the field is 4 bits wide, the maximum value it can store is 15, thus the maximum length of the header is 60 bytes.

Internet Header Length is the length of the internet header in 32 bit words, and thus points to the beginning of the data. Note that the minimum value for a correct header is 5.

Which means whatever value is stored in the IHL, it should be multiplied with 32 to get the total number of bits, or with 4 to get the total number of bytes.

Upvotes: 12

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