The IP header

The 20 bytes of the IPv4 header field by field, the decreasing TTL, the checksum, fragmentation and the MTU.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

IHL: what is it?
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A 4-bit field giving the header's length in 4-byte words; the minimum value is 5.
5 × 4 = 20 bytes, the header without options. It tells where the data begins: 20 bytes cannot be assumed, since options can stretch the header up to 60.
Here is a captured IPv4 header. What is the packet's remaining time to live (TTL)?

```text
45 00 00 3c 1c 46 40 00 40 06 b1 4b c0 a8 00 68 ac 10 00 0a
`
- 64
- 6
- 40
- 128
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64
The TTL is the 9th byte (offset 8): 0x40 = 64. The next byte, 0x06, is the protocol, and 40 is the hexadecimal value read as decimal.
The IHL counts the header's length in words of … bytes: the minimum value 5 gives … bytes.
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The IHL counts the header's length in words of 4 bytes: the minimum value 5 gives 20 bytes.
Four bits allow 15 at most, which is 60 header bytes; the 40 bytes above 20 are options, rare today.