Networking Fundamentals PDF

Chapter 11: Networking Fundamentals 553 We next took a look at the various technologies that allow us to build networks from the ground up. There are three types of LANs that you need to remember f...

Chapter 11: Networking Fundamentals 553 We next took a look at the various technologies that allow us to build networks from the ground up. There are three types of LANs that you need to remember for the exam: Ethernet, Token Ring, and FDDI. Recall that LANs are limited in geographical scope but can be linked together using technologies like dedicated links, frame relay, SONET, and ATM to form MANs and WANs. Once you extend past the local area (and oftentimes even within it), you’ll need routers to break up broadcast domains and link together the pieces of your MAN or WAN. Quick Review A protocol is a set of rules that dictates how computers communicate over networks. The application layer, layer 7, has services and protocols required by the user’s applications for networking functionality. The presentation layer, layer 6, formats data into a standardized format and deals with the syntax of the data, not the meaning. The session layer, layer 5, sets up, maintains, and breaks down the dialog (session) between two applications. It controls the dialog organization and synchronization. PART IV The transport layer, layer 4, provides end-to-end transmissions. The network layer, layer 3, provides routing, addressing, and fragmentation of packets. This layer can determine alternative routes to avoid network congestion. Routers work at the network layer, layer 3. The data link layer, layer 2, prepares data for the network medium by framing it. This is where the different LAN and WAN technologies work. The physical layer, layer 1, provides physical connections for transmission and performs the electrical encoding of data. This layer transforms bits to electrical signals. A network topology describes the arrangement of computers and devices. In a bus topology, a single cable runs the entire length of the network and nodes attach to it through drop points. In a star topology, all nodes connect to a central device such as a switch using a dedicated link. In a mesh topology, all nodes are connected to each other in a non-uniform manner that provides multiple paths to most or all the nodes on the network. A ring topology has a series of devices connected by unidirectional transmission links that form a closed loop and do not connect to a central system. Ethernet uses CSMA/CD, which means all computers compete for the shared network cable, listen to learn when they can transmit data, and are susceptible to data collisions. CISSP All-in-One Exam Guide 554 Token Ring, IEEE 802.5, is an older LAN implementation that uses a token- passing technology. FDDI is a LAN and MAN technology, usually used for backbones, that uses token-passing technology and has redundant rings in case the primary ring goes down. TCP/IP is a suite of protocols that is the de facto standard for transmitting data across the Internet. TCP is a reliable, connection-oriented protocol, while IP is an unreliable, connectionless protocol. Data is encapsulated as it travels down the network stack on the source computer, and the process is reversed on the destination computer. During encapsulation, each layer adds its own information so the corresponding layer on the destination computer knows how to process the data. Two main protocols at the transport layer are TCP and UDP. UDP is a connectionless protocol that does not send or receive acknowledgments when a datagram is received. It does not ensure data arrives at its destination. It provides “best-effort” delivery. TCP is a connection-oriented protocol that sends and receives acknowledgments. It ensures data arrives at the destination. ARP translates the IP address into a MAC address (physical Ethernet address), while RARP translates a MAC address into an IP address. ICMP works at the network layer and informs hosts, routers, and devices of network or computer problems. It is the major component of the ping utility. DNS resolves hostnames into IP addresses and has distributed databases all over the Internet to provide name resolution. Altering an ARP table so an IP address is mapped to a different MAC address is called ARP poisoning and can redirect traffic to an attacker’s computer or an unattended system. Routers link two or more network segments, where each segment can function as an independent network. A router works at the network layer, works with IP addresses, and has more network knowledge than bridges, switches, or repeaters. IPv4 uses 32 bits for its addresses, whereas IPv6 uses 128 bits; thus, IPv6 provides more possible addresses with which to work. NAT is used when organizations do not want systems to know internal hosts’ addresses, and it enables organizations to use private, nonroutable IP addresses. Subnetting allows large IP address ranges to be divided into smaller, logical, and easier-to-maintain network segments. Dedicated links are usually the most expensive type of WAN connectivity method because the fee is based on the distance between the two destinations rather than on the amount of bandwidth used. T1 and T3 are examples of dedicated links. Frame relay and X.25 are packet-switched WAN technologies that use virtual circuits instead of dedicated ones.