Network

Networks are classified by scope (personal, local, metropolitan and wide area networks), by topology (bus, ring, star, mesh) and by how they share capacity. Telephone networks traditionally used circuit switching, reserving a fixed path and bandwidth for the whole call; computer networks use packet switching, where data is chopped into packets that share links through statistical multiplexing. Packet switching uses capacity far more efficiently for bursty traffic, at the price of variable delay and possible loss when queues overflow. Physically, a modern office LAN is a switched star: every device has its own link to a switch port, and switches are linked to each other and to routers.

Inside a LAN, the link layer does the delivery. Ethernet (IEEE 802.3) and Wi-Fi (IEEE 802.11) carry frames addressed by 48-bit MAC addresses. A switch learns which MAC address sits behind which port and stores this in its forwarding (CAM) table; frames to unknown addresses and broadcasts are flooded to all ports in the same broadcast domain. VLANs (IEEE 802.1Q) split one physical switch fabric into several logical broadcast domains, and routers connect those domains at the IP layer. ARP (RFC 826) glues the layers together by finding the MAC address that belongs to an IPv4 address, and the Spanning Tree Protocol prevents the forwarding loops that redundant cabling would otherwise create.

Performance is described by bandwidth (the link's capacity), throughput (what is actually achieved), latency (the sum of propagation, transmission, queuing and processing delay), jitter and packet loss. The bandwidth-delay product states how much data must be in flight to fill a path: at 1 Gbit/s and a 10 ms round-trip time it is about 1.25 MB, which is why transport protocols need large windows on fast, long paths.

The local network was built on trust, and many classic attacks exploit that. ARP spoofing lets a host on the same segment claim another host's IP address and place itself in the middle; MAC flooding fills a switch's table so that it floods traffic to every port; rogue DHCP servers hand out a malicious gateway or DNS server. Managed switches counter these with dynamic ARP inspection, DHCP snooping and port security, and IEEE 802.1X lets a switch or access point require authentication before a device gets network access at all. At the design level, a flat network where every device can reach every other lets one compromised laptop scan and attack the whole organisation, which is why network segmentation, up-to-date network diagrams and an inventory of connected devices are basic controls.