Ipv4 Frame Format

1. Version (4 bits)

Indicates the IP version being used. An Ipv4 packet -> 4 (in binary 0100) An Ipv6 packet -> 6 (in binary 0110)

2. IHL (Internet Header Length) (4 bits)

Indicates where the header ends and where the actual user data begins It measures in groups of 4-byte words. Its normal value is 5 (5 x 4 = 20 bytes)

3. Type of Service (ToS) / DSCP (8 bits)

Used for QoS. It alows packets to be marked with a specific priority

4. Total Length (16 bits)

Specifies the entire size of the complete packet. (IP Header + Payload)

5. Identification (16 bits)

“ID” or serial number of the original packet. If the packet gets fragmented, all fragments will carry this exact same number.

6. Flags (3 bits)

The first bit is always set to 0.
DF Bit (Don’t Fragment): If set to 1, prohibits routers from fragmenting the packet.
MF Bit (More Fragments): If set to 1, it warns the receiver that more pieces are on the way. If set to 0, it means this is the last fragment.

7. Fragment Offset (13 bits)

Indicates the exact position of that fragment within the original packet.

8. TTL (Time to Live) (8 bits)

A security counter to prevent a packet from looping indefinitely around the internet if there is a routing loop. Max value 255.

9. Protocol (8 bits)

Indicates which Layer 4 (Transport) protocol is encapsulated inside the IP packet. 1 -> ICMP 6 -> TCP 17 -> UDP 89 -> OSPF

10. Header Checksum (16 bits)

The quality control for the IP header. Every single router along the path must recalculate this field.