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Network Architecture Basics
Before you can truly understand network security, you have to first understand network architecture. Although this book is not intended to serve as a network primer, this section is a quick review of network concepts and terms. I will be referring to these terms often and it will help you to have a basic understanding of the TCP/IP protocol. If you are already well-schooled in network topologies, then you can skip over this section and jump straight into the tools.
As you may know, every network design can be divided into seven logical parts, each of which handles a different part of the communication task. This seven-layered design is called the OSI Reference Model. It was created by the International Standards Organizations (ISO) to provide a logical model for describing network communications, and it helps vendors standardize equipment and software. Figure 3.1 shows the OSI Reference Model and gives examples of each layer.
Figure 3.1. The OSI Reference Model
This layer is the actual physical media that carries the data. Different types of media use different standards. For example, coaxial cable, unshielded twisted pair (UTP), and fiber optic cable each serve a different purpose: coaxial cable is used in older LAN installations as well as Internet service through cable TV networks, UTP is generally used for in-house cable runs, while fiber optic is generally used for long-haul connections that require a high load capacity.
This layer relates to different pieces of network interface hardware on the network. It helps encode the data and put it on the physical media. It also allows devices to identify each other when trying to communicate with another node. An example of a data link layer address is your network card's MAC address. (No, the MAC address doesn't have anything to do with Apple computers; it's the Medium Access Control number that uniquely identifies your computer's card on the network.) On an Ethernet network, MAC addresses are the way your computer can be found. Corporations used many different types of data link standards in the 1970s and 80s, mostly determined by their hardware vendor. IBM used Token Ring for their PC networks and SNA for most of their bigger hardware, DEC used a different standard, and Apple used yet another. Most companies use Ethernet today because it is widespread and cheap.
This layer is the first part that you really see when interacting with TCP/IP networks. The network layer allows for communications across different physical networks by using a secondary identification layer. On TCP/IP networks, this is an IP address. The IP address on your computer helps get your data routed from place to place on the network and over the Internet. This address is a unique number to identify your computer on an IP-based network. In some cases, this number is unique to a computer; no other machine on the Internet can have that address. This is the case with normal publicly routable IP addresses. On internal LANs, machines often use private IP address blocks. These have been reserved for internal use only and will not route across the Internet. These numbers may not be unique from network to network but still must be unique within each LAN. While two computers may have the same private IP address on different internal networks, they will never have the same MAC address, as it is a serial number assigned by the NIC manufacturer. There are some exceptions to this (see the sidebar Follow the MAC), but generally the MAC address will uniquely identify that computer (or at least the network interface card inside that computer).
This level handles getting the data packet from point A to point B. This is the layer where the TCP and UDP protocols reside. TCP (Transmission Control Protocol) basically ensures that packets are consistently sent and received on the other end. It allows for bit-level error correction, retransmission of lost segments, and fragmented traffic and packet reordering. UDP (User Datagram Protocol) is a lighter weight scheme used for multimedia traffic and short, low-overhead transmissions like DNS requests. It also does error detection and data multiplexing, but does not provide any facility for data reordering or ensured data arrival. This layer and the network layer are where most firewalls operate.
The session layer is primarily involved with setting up a connection and then closing it down. It also sometimes does authentication to determine which parties are allowed to participate in a session. It is mostly used for specific applications higher up the model.
This layer handles certain encoding or decoding required to present the data in a format readable by the receiving party. Some forms of encryption could be considered presentation. The distinction between application and session layers is fine and some people argue that the presentation and application layers are basically the same thing.
This final level is where an application program gets the data. This can be FTP, HTTP, SMTP, or many others. At this level, some program handling the actual data inside the packet takes over. This level gives security professionals fits, because most security exploits happen here.
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