IPv4 header format

The IPv4 header is 20 bytes minimum (up to 60 bytes with options), in 32-bit words:

 0       4       8              16      19                    31
+-------+-------+---------------+-----------------------------+
|Version|  IHL  | DSCP/ToS (8)  |        Total Length (16)     |
+-------+-------+---------------+-----+-----------------------+
|      Identification (16)      |Flags|   Fragment Offset (13) |
+---------------+---------------+-----+-----------------------+
|  TTL (8)      | Protocol (8)  |      Header Checksum (16)    |
+---------------+---------------+-----------------------------+
|                  Source IP Address (32)                      |
+--------------------------------------------------------------+
|                Destination IP Address (32)                   |
+--------------------------------------------------------------+
|                Options (variable) + Padding                  |
+--------------------------------------------------------------+

Fields

Field	Size	Description
Version	4 bits	IP version = 4
IHL (Header Length)	4 bits	Header length in 32-bit words (min 5 = 20 bytes, max 15 = 60 bytes)
DSCP / Type of Service	8 bits	Priority and quality-of-service marking (DSCP + ECN)
Total Length	16 bits	Length of the whole datagram (header + data) in bytes; max 65,535
Identification	16 bits	Unique ID of the datagram; all fragments of one datagram carry the same ID, used for reassembly
Flags	3 bits	Bit 0 reserved; DF (Don't Fragment); MF (More Fragments: 1 = more follow, 0 = last fragment)
Fragment Offset	13 bits	Position of this fragment's data in the original datagram, in units of 8 bytes
TTL (Time To Live)	8 bits	Hop limit, decremented by each router; the packet is discarded at 0 (prevents loops)
Protocol	8 bits	Upper-layer protocol: 1 = ICMP, 6 = TCP, 17 = UDP
Header Checksum	16 bits	Error check on the header only (recomputed at each router as TTL changes)
Source Address	32 bits	IP address of the sender
Destination Address	32 bits	IP address of the receiver
Options	0–40 bytes	Optional: record route, timestamp, security, source routing
Padding	variable	Zeros to make the header a multiple of 32 bits

Fragmentation

Why: every link has a Maximum Transmission Unit (MTU), the largest frame payload it can carry (Ethernet = 1500 bytes). If a datagram is larger than the MTU of the outgoing link, the router splits it into smaller fragments.

Rules:

1. Each fragment gets its own IP header (a copy of the original with changed fields).
2. Identification is copied unchanged, so the destination can group fragments.
3. MF = 1 for all fragments except the last (MF = 0).
4. Fragment offset = (byte position of the fragment's data) / 8, so every fragment except the last must carry data that is a multiple of 8 bytes.
5. Total length and checksum are recomputed for each fragment.
6. If DF = 1 and fragmentation is required, the packet is dropped and an ICMP "Fragmentation needed" message is sent back.
7. Reassembly happens only at the final destination, not at intermediate routers. If any fragment is lost, the whole datagram is discarded after a timeout.

Example

A datagram of 4000 bytes (20-byte header + 3980 data) crosses a link with MTU = 1500.

Max data per fragment = 1500 − 20 = 1480 bytes (1480 / 8 = 185, a whole number, good).

Fragment	Data bytes	Total length	Offset (bytes)	Fragment Offset field	MF
1	1480	1500	0	0	1
2	1480	1500	1480	185	1
3	1020	1040	2960	370	0

Check: 1480 + 1480 + 1020 = 3980 data bytes.

Drawbacks: extra header overhead, extra router processing, and higher loss probability. IPv6 removes router fragmentation; only the source fragments, after Path MTU Discovery.