IP Packets

An IP packet is a fundamental unit of data transmission in IP networks, including the internet. It is a protocol data unit (PDU) at the Network layer (Layer 3) in the OSI model, and it contains information necessary to route the packet across different networks to its destination.

An IP packet is made up of two parts: the header and the payload. The header contains information about the packet, including the source and destination IP addresses, the protocol used in the payload, the time to live (TTL), and other control information. The payload contains the actual data being transmitted.

The structure of the IP header varies depending on the version of the IP protocol being used. In IPv4, the header is 20 bytes long (excluding any optional fields), while in IPv6, the header is 40 bytes long. Some of the key fields in the IP header include:

Version: Indicates whether the packet is an IPv4 or IPv6 packet.

Header length: Specifies the length of the IP header in 32-bit words.

Differentiated Services (DSCP): Used for Quality of Service (QoS) to prioritize different types of traffic.

Time to Live (TTL): Specifies the maximum number of hops (routers) the packet can pass through before being discarded.

Protocol: Specifies the protocol used in the payload (e.g., TCP, UDP, ICMP).

Source and destination IP addresses: Specifies the IP addresses of the source and destination devices.

Once an IP packet is created, it is then sent to the Data Link layer (Layer 2) for encapsulation in a Data Link layer frame, which includes the Data Link layer addressing information (e.g., MAC addresses). The resulting packet is then transmitted across the network to its destination, where it is de-encapsulated and processed by the recipient device.