A data packet is the unit of data transmitted during TCP/IP communication. A single message is divided into multiple data blocks, which are called packets, containing the address information of both the sender and receiver. These packets then travel along different paths across one or more networks and are reassembled at their destination.
I. Structure of a Data Packet
The structure of a data packet is very complex. Here, we’ll focus on understanding its key components, which is essential for grasping the communication principles of the TCP/IP protocol.
A data packet mainly consists of a destination IP address, a source IP address, and payload data. The structure is very similar to how we write a letter. The destination IP address specifies whom the packet is for, equivalent to the recipient’s address; the source IP address indicates where the packet came from, equivalent to the sender’s address; and the payload data is equivalent to the letter’s content.
II. It is precisely because of this structure that computers installed with the TCP/IP protocol can communicate with each other. When we use a network based on the TCP/IP protocol, what is actually being transmitted across the network are data packets.
Understanding data packets is critically important for network management and network security. The simple act of opening a webpage involves you first sending data packets to the website. After receiving them, based on the IP address in the data packets you sent, it returns webpage data packets to you. In other words, browsing the web is essentially an exchange of data packets.
III. Packet filtering, sometimes also called static packet filtering, controls network access by analyzing incoming and outgoing data packets and passing or blocking them based on predefined criteria. When a router forwards or rejects packets according to filtering rules, it acts as a packet filter.
When a packet arrives at a packet-filtering router, the router extracts certain information from the packet header and decides, based on the filtering rules, whether the packet should be passed or dropped. Packet filtering operates at the Network Layer of the Open Systems Interconnection model, or the Internet Layer of TCP/IP.
IV. As Layer 3 devices, packet-filtering routers use rules based on source and destination IP addresses, source and destination ports, and the packet’s protocol to decide whether to permit or deny traffic. These rules are defined using Access Control Lists (ACLs). An ACL is a sequential list of permit or deny statements applied to IP addresses or upper-layer protocols. An ACL can extract the following information from a packet header, test