Computer Networks and Internet Technologies
Topologies and scope
Physical layouts include bus, star, ring, mesh, tree and hybrid; star dominates LANs because one cable failure affects only one node. Networks are classed as PAN, LAN, MAN and WAN, with VPNs providing secure tunnels over public links. Architectures are client-server or peer-to-peer.
OSI and TCP/IP
The OSI model has seven layers: Physical, Data Link, Network, Transport, Session, Presentation and Application. TCP/IP collapses these into network access, internet, transport and application. Devices: a hub is layer 1 and repeats to all ports, a switch is layer 2 and forwards by MAC address, a router is layer 3 and forwards by IP address, and a gateway translates between protocols. Protocols include IP, TCP (connection-oriented, reliable), UDP (connectionless, fast), HTTP/HTTPS, FTP, SMTP, POP3, IMAP, DNS, DHCP and ARP.
Media and addressing
Guided media are twisted pair (UTP/STP with RJ-45), coaxial cable and fibre (immune to electromagnetic interference). Unguided media are radio, microwave, satellite and infrared. An IPv4 address is 32 bits in four octets; private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16.
Trap: subnetting and device roles. A hub creates one collision domain, a switch separates collision domains, and a router separates broadcast domains.
Worked example: 192.168.10.0/26 gives mask 255.255.255.192, a block size of 64 and usable hosts of 2^6 − 2 = 62, with subnets starting at .0, .64, .128 and .192. Rule of thumb: usable hosts = 2^(32 − prefix) − 2, because the network and broadcast addresses are reserved.