packet
0-3: "Version"
4-7: "IHL"
8-15: "Type Of Service"
16-31: "Total Length"
32-47: "Identification"
48-50: "Flags"
51-63: "Fragment Offset"
64-71: "Time to Live"
72-79: "Protocol"
80-95: "Header Checksum"
96-127: "Source Address"
128-159: "Destination Address"
160-183: "Options"
184-191: "Padding"
Field Length Description
Version 4 bits 0100 for IPv4, 0110 for IPv6IHL — Internet Header Length4 bits Length of the header in 32-bit words. Minimum value is 5 (no options, no padding). ToS — Type of Service8 bits Quality of Service. Now used for DSCP. Total Length 16 bits Total packet size in bytes (header + data). 16 bits × 8 = max packet size of 65,535 bytes. Identification 16 bits Used to uniquely identify fragmented packets to add reassembly. Flags 3 bits always 0, May Fragment, More Fragments Fragment Offset 13 bits Where the fragment belongs. Units of 8 octets (64 bits). First fragment is set to 0. TTL — Time to Live8 bits Prevents routing loops. Each router decrements by 1; packet is discarded at 0. Protocol 8 bits What the packet encapsulates, Ex: 1 = ICMP, 6 = TCP, 17 = UDP, 88 = EIGRP, 89 = OSPF. Header Checksum 16 bits Covers the IP header only (not data). Recomputed at each device that processes the IP header. Source Address 32 bits The SA — IP address of the sending host. Destination Address 32 bits The DA — IP address of the destination host. IP Options Variable Loose/Strict Source Routing, Record Route, Timestamp. Mostly unused, historical. Padding Variable Ensures the header ends on a 32-bit boundary.
Flags: 3 bits
Various Control Flags.
Bit 0: reserved, must be zero
Bit 1: (DF) 0 = May Fragment, 1 = Don't Fragment.
Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.
0 1 2
+---+---+---+
| | D | M |
| 0 | F | F |
+---+---+---+