IPv4 Protocol, IPv4 header and fields of IPv4 header

IPv4 Protocol (Internet Protocol - Version 4)

The IPv4 Protocol implements two basic functions of network traffic; 1) Logical addressing / Routing of IPv4 datagram packets and 2) Fragmentation and Reassembly of IPv4 Datagrams.

1) Logical addressing / Routing of IPv4 datagram packets: Routing is the process of selecting best path / paths in a network along which to send the IPv4 datagrams efficiently. The Network Layer uses the IPv4 addresses (also called as logical addresses) for communication. IPv4 addresses (also called as logical addresses) are 32-bit binary numbers represented in 4 octets (or bytes). If you are using IPv4 in your network (another choice is IPv6), Routers use IPv4 addresses to route IPv4 datagrams from one LAN to another LAN.

2) Fragmentation and Reassembly of IPv4 Datagrams: To understand what is IPv4 Datagram Fragmentation, first we should know the term MTU (Maximum Transmission Unit). MTU is the size (in bytes) of the largest packet or frame that can pass through a specific device or NIC card. While travelling through the network to reach the destination, the Internet Protocol Version 4 (IPv4) datagrams may need to traverse different networks with heterogeneous MTUs. When a datagram is larger than the MTU of the network it needs to traverse, it is divided into smaller fragments and are sent separately. At the destination computer the fragmented IPv4 Datagram is reconstructed and this process in called reassembly.

More about Fragmentation and Reassembly of IPv4 packets are explained at the bottom part of this tutorial lesson.

IPv4 Header

Following image shows the structure and the fields of IPv4 header.

IPv4 Header Structure

"Version" field of IPv4 header

"Version" is 4-bit field indicates which version of Internet Protocol (IP) is being used. Here we are discussing about IPv4. The binary pattern for IPv4 is 0100. The possible values for Version field are as shown in the table below. IPv4 is moving away slowly from industry for IPv6.

IP Version Description
0 Reserved
1-3 Unassigned
4 IP Version 4
5 Stream IP Datagram mode (experimental protocol)
6 IP Version 6
8 The "P" internet protocol
10-14 Unassigned
15 Reserved


Note: The TP/IX, "P" Internet protocol, and TUBA were the major protocols considered for the replacement of IPv4, but the industry embraced "IP Version 6" as the replacement for "IP Version 4" all other protocols mentioned above only have historical status.

"IHL (Internet Header Length)" field of IPv4 header

"IHL (Internet Header Length)" field is 4-bits long binary number. This 4-bit IHL (Internet Header Length) field gives the "length of IPv4 header" in 32-bit words. Since IHL (Internet Header Length) is a 4-bit binary number, IHL field in IPv4 header can hold values from 0000 (0 in decimals) to 1111 (15 in decimals). Minimum required field size of IPv4 header is 20 bytes. Minimum required length of an IPv4 header is five 32-bit words (5*32 = 160 bits or 20 bytes). The length of required fields in IPv4 header is 20 bytes. Maximum possible is fifteen 32-bit words (15*32 = 480 bits or 60 bytes). The bit pattern of is IHL field in IPv4 header is 0101, which is equal to five in decimals.

Please refer below image to understand the meaning of 32-bit words of IPv4 header. Numbered at right-side are the required fields of IPv4 header.


"Type of Service (ToS)" field of IPv4 header

The "Type of Service (ToS)" field in the Internet Protocol (IPv4) header is an eight-bit length binary number field, which provides an indication of the Quality of Service (QoS) desired, such as precedence, delay, throughput, and reliability.

By using Quality of Service (QoS), network administrators can prioritize specific types of traffic on the network to provide better user experience. QoS (Quality of Service) can be used to manage network traffic with different priority levels for different types of traffic. QoS (Quality of Service) can be used prioritize resource-intensive traffic like Voice over IP (VoIP), Online Gaming, Video Conferencing, IP Television (IPTV) etc.

Please refer two images placed below to view a graphical represntation of 8-bit "Type of Service (ToS)" field in IPv4 header.




The first three bits (0 to 2) of "Type of Service (ToS)" field in IPv4 header indicates the "Precedence" value. The following are the possible combinations for the first three bits (0 to 2) of "Type of Service (ToS)" field in IPv4 header.

Bits Pattern Description
000 Routine
001 Priority
010 Immediate
011 Flash
100 Flash Override
110 Internetwork Control
111 Network Control


The fourth bit indicates "Delay". The bit value “0” indicates normal delay and the bit value "1" indicate low delay.

The fifth bit indicates "Throughput". The bit value “0” indicates normal throughput and the bit value "1" indicate high throughput.

The sixth bit indicate "Reliability". The bit value “0” indicates normal reliability and the bit value "1" indicate high reliability.

The seventh bit indicates "Cost". The bit value "0" indicates "normal monetary cost" and the bit value "1" indicates "minimize monetary cost".

The eight bit is reserved for future use and is termed as MBZ (Must be Zero)

The four bits which represent the ToS values can be summarized as below.

ToS Code Meaning
1000 Minimum Delay
0100 Maximum throughput
0010 Maximum Reliability
0001 Minimum monetary cost
0000 Normal Service


"Total Length" field of IPv4 header

The "Total Length" in IPv4 header is a 16-bit field which identifies the length (in bytes), of the IPv4 datagram. Total Length includes the length of IPv4 header and the Data it carries. The minimum-length of an IPv4 Datagram is 20 bytes (The minimum size of an IP header is 20 bytes and this is the case of an IPv4 header carrying no data) and the maximum is 65,535 bytes (maximum possible value for a 16-bit number is 65,535).

"Identification" field of IPv4 header

The "Identification" field in the IPv4 header is a 16 bits field which indicates an identifying value assigned by the sender to aid in assembling the fragments of an IPv4 Datagram. When a Datagram is fragmented in to multiple Datagrams, IPv4 give all the fragments the same identification number and this number is used to identify IPv4 fragments at the receiving side. More about IPv4 fragmentation is given below.

"Flags" field of IPv4 header

The three bit "Flags" field in IPv4 header indicates fragmentation possibilities. The first bit is unused and should always have a value of zero. The next bit is called the DF (Don't Fragment) flag. DF flag set to "0" indicate that the IPv4 Datagram can be fragmented and DF set to 1 indicate "Don't Fragment" the IPv4 Datagram. The next bit is the MF (More Fragments) flag, which indicates that more fragments are on the way. When MF is set to 0, no more fragments need to be sent or the IPv4 Datagram was never fragmented.

Please refer below image to view a graphical represntation of 3-bit "Flags" field in IPv4 header.


"Fragment Offset" field of IPv4 header

"Fragment Offset" field of IPv4 header indicates where in the actual IPv4 Datagram this fragment belongs. The fragment offset is measured in units of 8 octets (64 bits). The first fragment has offset zero. To understand what is IPv4 fragmentation, refer below image.

IP Datagram Fragmentation

IPv4 Datagram Fragmentation

Network 1 and Network 3 has MTUs of 1500 bytes each and the MTU of Network 3 is only 1000 bytes. If Machine 1 sends a 1,500 byte Datagram (20-byte header and 1,480 bytes of data) to Machine 2, Router 1 must fragment the Datagram into two fragments, since the MTU for the Network 2 is only 1000 bytes.

1) The first fragment will contain 20 bytes IPv4 header and 976 bytes of data. Also note that the Fragment offset is set to "0" (Since first fragment) and the MF (More Fragments) flag is set to "1".

2) The second fragment will contain a 20 byte IPv4 header and 504 bytes of data. The fragment offset is set to 122 (976/8 = 122) and the MF (More Fragments) flag is set to "0". When the IPv4 Datagram is fragmented to two, there is an additional 20 bytes of transfer which is the IPv4 Datagram header size of the second fragment.

"Time to Live (TTL)" field of IPv4 header

"Time to Live (TTL)" is an 8-bit field and Time to Live (TTL) indicates the amount of time in seconds (or router hops) that the IPv4 Datagram can survive before being discarded. Every router examines and decrements this field by at least 1, or by the number of seconds the IPv4 Datagram is delayed inside the router. The Internet Protocol Version 4 (IPv4) Datagram is discarded when this field reaches zero.

In computer networking, Time to Live (TTL) field value is typically used to prevent routing loops.

"Protocol" field of IPv4 header

The 8-bit "Protocol" field indicates the protocol that will receive the data payload. For example, ICMP (Internet Control Message Protocol) has a Protocol number of 1, TCP (Transmission Control Protocol) has a Protocol number of 6, UDP (User Datagram Protocol) has a Protocol number of 17, RDP (Reliable Data Protocol) has a Protocol number of 27, ESP (Encapsulating Security Payload) has a Protocol number of 50 etc.

The Protocol field in IPv4 header is used to demultiplex an IPv4 packet to a Transport layer (Layer 4) protocol. Following table lists important Protocol numbers. You may find latest Protocol Number Assignments from IANA Protocol Number Assignments page.

Field Value Protocol Description
0 IPv6 Hop-by-Hop Option.
1 ICMP (Internet Control Message Protocol)
2 IGMP (Internet Group Management Protocol)
RGMP (Router-port Group Management Protocol)
3 GGP (Gateway to Gateway Protocol)
4 IPv4 in IP encapsulation
5 ST (Internet Stream Protocol)
6 TCP (Transmission Control Protocol)
7 CBT (Core Based Trees)
8 EGP (Exterior Gateway Protocol)
9 IGRP (Interior Gateway Routing Protocol - Cisco)
10 BBN- RCC-MON (BBN RCC Monitoring )
11 NVP (Network Voice Protocol)
12 Xerox PUP
13 ARGUS (deprecated)
14 EMCON (Emission Control Protocol)
15 XNET Cross Net Debugger
16 Chaos (Chaosnet)
17 UDP (User Datagram Protocol)
18 TMux (Transport Multiplexing Protocol)
19 DCN Measurement Subsystems
20 HMP (Host Monitoring Protocol)
21 Packet Radio Measurement
23 Trunk-1
24 Trunk-2
25 Leaf-1
26 Leaf-2
27 RDP (Reliable Data Protocol)
28 IRTP (Internet Reliable Transaction Protocol)
29 ISO Transport Protocol Class 4.
30 NETBLT (Network Block Transfer)
31 MFE Network Services Protocol
32 MERIT Internodal Protocol
33 Sequential Exchange Protocol
34 Third Party Connect Protocol
35 IDPR (Inter-Domain Policy Routing Protocol)
36 XTP (Xpress Transfer Protocol)
37 Datagram Delivery Protocol
38 IDPR-CMTP (IPDR Control Message Transport Protocol)
39 TP++ (TP++ Transport Protocol)
40 IL Transport Protocol
41 IPv6 over IPv4 (6in4)
42 SDRP (Source Demand Routing Protocol)
43 IPv6 Routing header
44 IPv6 Fragment header
45 IDRP (Inter-Domain Routing Protocol)
46 RSVP (Reservation Protocol)
47 GRE (General Routing Encapsulation)
48 MHRP (Mobile Host Routing Protocol)
49 BNA (Burroughs Network Architecture)
50 ESP (Encapsulating Security Payload)
51 AH (Authentication Header)
52 Integrated Net Layer Security TUBA
53 SWIPE (IP with Encryption (deprecated))
54 NARP (NBMA Address Resolution Protocol)
55 Minimal Encapsulation Protocol
56 TLSP (Transport Layer Security Protocol using Kryptonet key management)
57 SKIP (Simple Key-Management for Internet Protocol)
58 ICMPv6 (Internet Control Message Protocol for IPv6)
MLD (Multicast Listener Discovery)
59 IPv6 No Next Header
60 Destination Options for IPv6
61 Any host internal protocol
63 Any local network
64 SATNET and Backroom EXPAK
65 Kryptolan
66 MIT Remote Virtual Disk Protocol
67 Internet Pluribus Packet Core
68 Any distributed file system
69 SATNET Monitoring
70 VISA Protocol
71 Internet Packet Core Utility
72 Computer Protocol Network Executive
73 Computer Protocol Heart Beat
74 Wang Span Network
75 Packet Video Protocol
76 Backroom SATNET Monitoring
77 SUN ND PROTOCOL-Temporary
78 WIDEBAND Monitoring
80 ISO Internet Protocol
81 VMTP (Versatile Message Transaction Protocol)
84 TTP
86 Dissimilar Gateway Protocol
87 TCF
88 EIGRP (Enhanced Interior Gateway Routing Protocol - Cisco)
89 OSPF (Open Shortest Path First Routing Protocol)
MOSPF (Multicast Open Shortest Path First)
90 Sprite RPC Protocol
91 Locus Address Resolution Protocol
92 MTP (Multicast Transport Protocol)
93 AX.25 (AX.25 Frames)
94 IPIP (IP-within-IP Encapsulation Protocol)
95 MICP (Mobile Internetworking Control Protocol, depricated)
96 SCC-SP (Semaphore Communications Sec. Pro.)
97 ETHERIP (Ethernet-within-IP Encapsulation)
98 ENCAP (Encapsulation Header)
99 Any private encryption scheme
100 GMTP
101 IFMP (Ipsilon Flow Management Protocol)
102 PNNI over IP
103 PIM (Protocol Independent Multicast)
104 ARIS (Aggregate Route IP Switching Protocol)
105 SCPS
106 QNX
107 A/N (Active Networks)
108 IPComp (IP Payload Compression Protocol)
109 SNP (Sitara Networks Protocol)
110 Compaq-Peer (Compaq Peer Protocol)
111 IPX-in-IP (IPX in IP)
112 VRRP (Virtual Router Redundancy Protocol)
113 PGM (PGM Reliable Transport Protocol)
114 any 0-hop protocol
115 L2TP (Level 2 Tunneling Protocol)
116 DDX (D-II Data Exchange)
117 IATP (Interactive Agent Transfer Protocol)
118 STP (Schedule Transfer Protocol)
119 SRP (SpectraLink Radio Protocol)
120 UTI (Universal Transport Interface Protocol)
121 SMP (Simple Message Protocol)
122 SM (Simple Multicast Protocol)
123 PTP (Performance Transparency Protocol)
124 ISIS over IPv4
125 FIRE (Flexible Intra-AS Routing Environment )
126 CRTP (Combat Radio Transport Protocol)
127 CRUDP (Combat Radio User Datagram)
128 SSCOPMCE (Service-Specific Connection-Oriented Protocol in a Multilink and Connectionless Environment)
129 IPLT
130 SPS (Secure Packet Shield)
131 PIPE (Private IP Encapsulation within IP)
132 SCTP (Stream Control Transmission Protocol)
133 Fibre Channel
135 Mobility Header
135 Mobility Header
137 MPLS-in-IP
253 Testing
254 Testing
255 Reserved

"Header Checksum" field of IPv4 header

The "Header Checksum" field holds a 16-bit calculated value to verify the validity of the header only. This field is recomputed in every router as the TTL field decrements.

"Source and Destination IPv4 Address" fields of IPv4 header

Source and Destination IPv4 Address fields are the most important fields of IPv4 header.

The "Source IP Address" is the 32-bit size IPv4 address of the device which sends this Internet Protocol (IPv4) Datagram.

The "Destination IPv4 Address" is the 32-bit size IPv4 address of the device which is going to receive this IPv4 Datagram.

"IP Options" field of IPv4 header

"IP Options" field supports a number of optional header settings primarily used for testing, debugging, and security.

"Padding" field of IPv4 header

The IPv4 Options field may vary in length. The "Padding" field provides additional zero bits so that the total header length is an exact multiple of 32 bits.


"Data" is the real IPv4 data payload and the actual size may vary. Data is not a part of the IPv4 header and it contains data generated by other protocols like TCP, UDP, ICMP, IGMP etc.

You have learned Internet Protocol Version 4 (IPv4), Internet Protocol Version 4 (IPv4) Datagram Header, different fields in an IPv4 header, and IPv4 Datagram fragmentation. The different fields of an IPv4 header are Version, IHL, Type of Service, Total Length, Flags, Fragment offset, TTL (Time to Live), Protocol, Header Checksum, Source IPv4 Address, Destination IPv4 Address, IP Options, Padding and the Data Payload. Click "Next" link below to continue.

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