IP headers play an essential role on the Internet. They ensure the data sent to the destination and received from it is complete, correct, and in order.
Cisco defines an IP header as something that “contains the information required to route data on the Internet and has the same format regardless of the type of data being sent.”
This article will cover everything you need to know about an IP header. Read on.
🔑 Key Takeaways
- IP headers include vital details such as source and destination addresses, packet length, IP version, and time-to-live (TTL), ensuring precise data transmission.
- Effective routing and delivery of packets are enabled by IP headers, promoting orderly data exchange across the Internet.
- IPv4 headers consist of 13 fields, whereas IPv6 headers comprise 8 fields—showcasing variations in size and address capacity.
- Collaborating with protocols like TCP, IP headers contribute to reliable and sequential data transmission.
- Familiarity with IP headers is crucial for optimizing data exchange, proficient packet handling, and mitigating transmission complications.
Understanding IP Header and How It Works
An IP header refers to the information included at the beginning of an IP packet.
This information includes details such as:
- IP version
- Packet length
- IP addresses (Source and Destination)
- Time-to-live
If you’re unfamiliar with IP packets, they are the tiniest messages exchanged on the web.
You can think of these packets as actual packages. A package contains two things: the content inside and the label outside.
The contents inside are the data you receive and send to another server. Meanwhile, the label outside is the header.
An IP packet header gives information to describe the data in the packet for accurate receiving, processing, and returning.
With the details, you can ensure proper and efficient sending and delivery.
| ⏳In a Nutshell The IP header holds vital information for IP packets—including IP version, packet length, IP addresses, and time-to-live. It ensures accurate packet handling and efficient delivery. |
IP Header Fields
You’ll find several header fields depending on the version of IP. Each header field includes essential information about the transferred data.
Since IPv4 and IPv6 have different headers, take a closer look at each one:
1. IPv4 Header Field
You can find 13 fields in an IPv4 header. Here’s what the IP address header format looks like on an IPV4:
To better understand IPv4 header fields, check out the table below:
| Field Name | Field Size | Function/s |
| Version | 4 bits | Describes the IP version used in the packet |
| Header length | 4 bits | Shows the header’s length because the size varies every time |
| TOS (Type of Service) | 8 bits | Explains the service necessary for the packet (some types are throughput, precedence, delay, and reliability) |
| Total Length | 16 bits | Determines the packet’s entire size (65,535 is the max limit) |
| Identification | 16 bits | Shows the identification number (only when a device cannot send the data into one packet) |
| Flag | 3 bits | Allows fragmentation on the packet (1st bit is 0; 2nd bit determines if it needs fragments; 3rd bit tells whether more fragments remain after the current one) |
| Offset | 13 bits | Shows which part of the original packet a fragment belongs |
| TTL or Time to Live | 8 bits | Tells the time or number of hops a packet can do before discarding |
| Protocol | 8 bits | Specifies the protocol’s upper layer and instructs the receiver on how to process the packet |
| Checksum | 16 bits | Checks the header’s validity at every hop (If the verification fails, the packet gets discarded.) |
| Source address | 32 bits | Shows the IP address of the source of the data or request |
| Destination address | 32 bits | Indicates the IP address of the destination of the data or request |
| Options (Optional Field) | 40 bits | Specifies options like the route the packets need to go through |
2. IPv6 Header Field
An IPv6 header only has 8 fields. The address size in this header is four times bigger than IPv4, but the header itself is only two times bigger.
Here’s what an IP address header format looks like on an IPV6:
Check out how each field works using the table below:
| Field Name | Field Size | Function/s |
| Version | 4 bits | Describes the IP version used in the packet |
| Traffic class | 8 bits | Same as the TOS field in an IPv4 header (The first 6 bits tell the service for the packet while the last 2 bits are for ECN) |
| Flow label | 20 bits | Labels the packets in the same group to ensure the data is in order (usually for real-time data like video or audio) |
| Payload length | 16 bits | Tells the packet’s size (with the highest value being 65,535) |
| Next header | 8 bits | Shows the first extension header type or determines the upper layer protocol (like UDP or TCP) |
| Hop limit | 8 bits | Determines the number of hops a packet can do before discarding |
| Source address | 128 bits | Shows the IP address of the source of the data or request |
| Destination address | 128 bits | Indicates the IP address of the destination of the data or request |
IPv4 and IPv6 Header
The headers for the two IP types differ in many ways. For a better comparison, here’s a side-by-side comparison between IPv4 and IPv6 headers:
IPv4 Header
IPv6 Header
Here’s a breakdown of their differences in every comparable aspect:
- Field Number
An IPv4 header includes 13 fields, while an IPv6 header only has 8.
Some fields, like the identification field in IP headers, are only available in an IPv4 header.
- Size
Despite having more fields, IPv4 headers are smaller—at around 20 to 60 bytes. It will only be more massive if you use the options field, which rarely happens.
Meanwhile, IPv6 headers are twice as big as IPV4 at around 40 bytes. Unlike the expandable IPv4 header, this one has a fixed size.
- Address Size
The main difference between the two headers is the space for the IP address size.
An IPv4 header has a 32-bit field, covering 4.29* 10^9 addresses. On the contrary, an IPv6 header has a 128-bit field, which can cover 3.4 * 10^38 addresses.
- Use of checksums or flags
The IPv4 header has a field for flags or checksums, while the IPv6 header has none. It means IPv6 does not use such fields.
Here’s a table to summarize the differences between the headers of the two versions of IP:
| Aspect | IPv4 Header | IPv6 Header |
| Field Number | 13 fields | 8 fields |
| Size | Smaller (20-60 bytes) | Larger (around 40 bytes) |
| Address Size | 32-bit field (covers 4.29*10^9 addresses) | 128-bit field (covers 3.4*10^38 addresses) |
| Use of Checksums or Flags | Includes field for flags or checksums | None |
TCP Header vs. IP Header
Transmission Control Protocol (TCP) works with IP to show how data exchange happens on the web.
With different tasks and purposes, the headers of TCP and IP differ from each other.
To understand their difference, here’s a comparison:
TCP Header
IP Header
Here are the main differences between the TCP and IP headers:
- Field Number
A TCP header includes only 10 fields (with one optional field), while an IP header has 13.
However, when compared with an IPv6 header, a TCP has more fields since IP’s latest version only has 8.
- Header Size
TCP and IPv4 headers have the same size of 20 to 60 bytes.
Comparing a TCP header to an IPv6 header, TCP is smaller because IPv6 has 40 bytes.
- Purpose
The IP sends each packet to its destination. Meanwhile, TCP ensures the transmission is correct, complete, and orderly.
When combined, they work as TCP/IP.
- Options Field
TCP has a higher capacity in its options field, up to 320 bits.
Meanwhile, IPv4’s options field can only have up to 32 bits. Moreover, IPv6 has no optional field.
Below is a table to sum up the distinctions between the two headers:
| Aspect | TCP Header | IP Header |
| Field Number | 10 fields (with 1 optional field ) | 13 fields |
| Header Size | 20 to 60 bytes | 20 to 60 bytes |
| Purpose | Secures the quality of transmission | Sends packets to destination |
| Options Field | Up to 320 bits | Up to 32 bits (IPv4 only) |
Conclusion
IP headers contain details ensuring efficient, correct, and orderly data exchange.
With these, every piece of data you send and receive will be detailed and accurate.
As a result, the servers can understand the data, its source, and the process it needs. These headers have different layouts depending on the versions.
Nonetheless, every type of header shares the same purpose. It ensures the data exchange will be smooth, error-free, fast, and orderly.
FAQs
What is the header for TCP?
The TCP header is the protocol’s initial 24 bits. This contains the data sets and conditions of a TCP socket. It checks the communication between two endpoints of the protocol.
How long is an IP header?
The length of an IP header can range from a minimum of 20 bytes to a maximum of 60 bytes, which comes with 32-bit increments.
What are flags in TCP?
Flags in TCP represent the state of connection. It is also used to supply more information for troubleshooting or take control of a specific connection.
Timeline Of The Article
Sirisha is a networking enthusiast with a bachelor's degree in computer science and engineering. She has developed a keen interest in Virtual Private Networks (VPNs) and has gone the extra mile to gain firsthand experience and knowledge.