IP provides logical host-to-host delivery across heterogeneous links. IPv4 header is 20 bytes minimum (IHL field), with options rarely used. Routers decrement TTL, recompute checksum, and may fragment when MTU drops.

IPv4 header — click key rows
text
1
Ver(4) | IHL | DSCP/ECN | Total Length
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2
Identification | Flags | Fragment Offset
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3
TTL | Protocol | Header Checksum
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4
Source Address | Destination Address
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