Basic Networking

Basic Networking Concepts

Vocabulary

A network may be thought of as a collection of nodes and links between those nodes. Technically a network requires 3 or more nodes; a link between 2 nodes is just called a link, and a link between fewer than 2 nodes is nonsensical.

Nodes

A node is any endpoint of a link. It can be a computer, a switch, a hub, or any other device which can be attached to a link.

Links

A link is any connection between nodes. It can be a wire, a radio transmission, a laser beam, or something else which transmits signals between nodes. Nodes on a link have link-layer addresses that identify them to one another. A link may have 2 or more endpoints.

Duplex

A link that can both transmit and receive is a duplex link. If it can transmit and receive at the same time then it's a full-duplex link. If it can either transmit or receive at a given time, it is a half-duplex link.

Hosts

Some nodes are called hosts. A host is a computer with a name. This name is the computers hostname. The hostname is the name by which the computer is meant to be identified on the network. It does not always work that way, but that's what it's for.

Routes

With only two nodes, there can be only one link. If there is only one link, then it's obvious how to send data between the nodes. If Host A and Host B are endpoints of Link 1, then Host A is a neighbor to Host B. As soon as there are more than 2 nodes, then there must be 2 or more links- depending on the network's topology. Neighbors can "see" each other over their link. When there are multiple links, it is no longer obvious how to send data between nodes. If Host C is on the other end of Link 2 from Host B, then B and C are neighbors but A and C are not. Thus A cannot send any data to C unless B co-operates. The way B abets A in sending data to C is called routing. A has a piece of information called a route that tells it that if it wants to send data to C, it should send it to B.

OSI and TCP/IP Models

Networking takes place in a "stack" of semi-independent 'layers'. Data links work basically the same regardless of the physical medium used; similarly, networks don't care about the nature of the data links that make them up- otherwise internetworking across different link types wouldn't be possible.

There are two main "models" of network layering. They are the OSI model (7 layers, yumm) and the TCP/IP model (4 layers- simpler but just as tasty). The TCP/IP model is a simplified version of the OSI model which is specifically relevant to TCP/IP networks; the OSI model is more general, though pure implementations of layers 5 and 6 are rarely seen in the wild. Both models are theoretical and don't necessarily map to real layers of real networks, but they are helpful tools for understanding and for diagnosis.

TCP/IP Model Layers

Layer 1: Physical

The physical layer refers to the physical means of signaling, whether wire, radio, laser, microwaves, cans-n-string, or what have you.

Layer 1 components include cables, transmitters, hubs, bridges, repeaters, antennae, splitters, splices, punch-downs, modems and other fun things.

- Repeaters

- Hubs

Layer 2: Data Link

The data link layer is the layer responsible for signaling between neighboring nodes.

Ethernet Protocol

MAC addresses

LLC and MAC sublayers, ethernet frame structure, header and footer fields, frame check sequence.

- Bridge

- Switch

Layer 3: IP

IPv4 - IP address structure, classless vs. classful

IPv6

IPv4 Subnetting

Routing basics

Related services/protocols: DHCP, DNS, ARP, ICMP

Layer 4: Transport

TCP vs. UDP

Ports, sockets

NAT and port forwarding

Common port numbers for protocols

OSI Model Layers

Physical

Same as in TCP/IP model.

Link

Same as in TCP/IP model.

Network

Same as in TCP/IP model, except embracing network protocols other than IP.

Transport

Same as in TCP/IP model, except embracing transport protocols other than UDP or TCP.

Application

Common protocols, FTP, HTTP, SSH, HTTPS, POP3, SMTP, IMAP4, etc.

Protocols

Protocols are sets of rules that devices use to communicate with each other for different purposes. They can function at one or several layers of the OSI model, and can call on other protocols to perform services. For example, HTTP is an application layer protocol that defines how web browsers and web servers communicate, and it also uses TCP protocol at the transport layer to provide a reliable transfer of data. Ethernet can refer to both ethernet wiring, and how the data on those wires is formatted for sending to particular hosts on the same LAN. TCP/IP refers to a suite of protocols that most networked devices commonly use for communication between LANs.

Data Encapsulation

The protocols used to move data from one host to another on a LAN, between LANs, and finally to a particular program on the machine on the recieving end all use layers of encapsulation. We'll start with the way data is encapsulated on a LAN. A network card takes the data it is given by the operating system, and adds a header with address information for another device on the LAN. The data, and the header that was added are together called a frame. A frame is the protocol data unit (PDU) used by ethernet. Next, within that frame is another PDU called an IP packet with its own header, and more data inside that. When that frame reaches a router, the router strips off the ethernet header, and looks at the IP header. It then chooses an interface to forward the IP packet out of based on the information in its routing table. If the other interface is an ethernet jack leading to a different LAN, it will encapsulate the packet again into a new frame with a new header.