The Hop

TTL is a Byte Field

The IPv4 TTL value is a single exact byte in the header.

highlighted = computed this step

TTL is a header byte

The IP version 4 header includes a TTL byte. In the before header, TTL is 64.

TTL=64\text{TTL}=64
Before the hopThe before IPv4 header is decoded from exact bytes.Before header160 bits / 20 bytes0x4500003c1c4640004006b1e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1101000000000001101011000111100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl64protocol6->TCPheader-checksum0xb1e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clear

Find the TTL field

The strip shows TTL after the flags and fragment offset, before the protocol field.

TTL byte=0x40\text{TTL byte}=0x40
Before the hopThe before IPv4 header is decoded from exact bytes.Before header160 bits / 20 bytes0x4500003c1c4640004006b1e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1101000000000001101011000111100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl64protocol6->TCPheader-checksum0xb1e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clear

Protocol stays separate

The next byte is protocol 6, which names TCP here. TTL and protocol are neighboring fields, not the same field.

protocol=6\text{protocol}=6
Before the hopThe before IPv4 header is decoded from exact bytes.Before header160 bits / 20 bytes0x4500003c1c4640004006b1e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1101000000000001101011000111100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl64protocol6->TCPheader-checksum0xb1e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clear

Summary

TTL is an exact byte in the header. A router changes that byte at each hop.

before TTL=64\text{before TTL}=64
Before the hopThe before IPv4 header is decoded from exact bytes.Before header160 bits / 20 bytes0x4500003c1c4640004006b1e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1101000000000001101011000111100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl64protocol6->TCPheader-checksum0xb1e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clear