Part 6. Layer 4 and IP access control lists

Having covered several key Layer 2 and Layer 3 concepts in previous parts of this book, in part 6, we now move up to Layer 4: the Transport Layer. Whereas Layers 1, 2, and 3 are focused on carrying messages between hosts with the help of the switches and routers that form the network infrastructure, Layer 4 runs on top of those lower layers and is responsible for ensuring that messages are delivered to the correct application in an efficient and reliable manner. Chapter 22 of this book delves into the two primary protocols that operate at this layer—TCP and UDP—comparing and contrasting their features, benefits, and drawbacks.

Chapters 23 and 24 move our focus to IP access control lists (ACLs), an essential tool for network security, controlling the flow of traffic by selectively permitting and denying packets. Standard ACLs, discussed in chapter 23, enable traffic filtering based on source IP addresses, providing a basic level of security and traffic control. Chapter 24 covers extended ACLs, providing a chance to apply the Layer 4 knowledge acquired in chapter 22; extended ACLs enable traffic filtering based not only on source and destination IP addresses but also Layer 4 TCP/UDP port numbers and other parameters.