Layer 3 — Network

What an IP address actually is, how the 32 bits are structured, what subnetting means, and how routers use addresses to make forwarding decisions.

From the moment a device connects with no IP address to the instant it is fully configured — every packet of the exchange, explained.

How subnetting divides a large IP block into smaller networks, the mathematics of network addresses and broadcast addresses, CIDR notation, VLSM, and the step-by-step process of designing a subnet scheme.

How the Domain Name System translates human-readable hostnames into IP addresses — the hierarchy of DNS servers, the resolution process, record types, caching, and why DNS is often called the phonebook of the internet.

What ICMP is, how it carries error reports and network diagnostics, the most important message types including Destination Unreachable and Time Exceeded, and how ping and traceroute are built on top of ICMP.

How static routes work, when to use them over dynamic routing protocols, administrative distance, route summarization with static routes, floating static routes for backup paths, and the limitations that make them impractical at scale.

How RIP uses distance-vector routing to share routes through hop count, why its simplicity comes with fundamental limitations, and how RIPv2 improved on the original with CIDR support and authentication.

How OSPF builds a complete map of the network using link-state advertisements, elects designated routers, calculates shortest paths with Dijkstra's algorithm, and scales through hierarchical area design.

How BGP connects autonomous systems across the internet using path-vector routing, what attributes control route selection, the difference between iBGP and eBGP, and why BGP is the most important routing protocol you will ever encounter.

How Network Address Translation maps private IP addresses to public ones, how Port Address Translation multiplexes thousands of connections through a single public IP, why NAT exists, and what problems it creates for modern networking.

How IGMP allows hosts to join and leave IP multicast groups, how routers use IGMP to track group membership and avoid flooding multicast traffic to devices that don't want it, and how IGMP snooping brings that efficiency to the switch level.

EIGRP is Cisco's advanced distance-vector routing protocol — faster convergence than RIP, simpler than OSPF, and the default interior routing protocol in most Cisco enterprise deployments. The DUAL algorithm guarantees loop-free paths and instant failover to a pre-calculated backup route without triggering a full network recalculation.

Why IPv6 exists, how its 128-bit address space is structured, and the major address categories — global unicast, link-local, multicast, and special addresses.

IPv6 address assignment methods — EUI-64 interface IDs, Stateless Address Autoconfiguration (SLAAC), stateless and stateful DHCPv6, and Neighbor Discovery Protocol.

How IPv6 packets are routed — static IPv6 routes, OSPFv3 as the IPv6 routing protocol, and key differences from IPv4 routing.

How HSRP, VRRP, and GLBP create a virtual gateway that survives individual router failures.

How the IP header's DS field classifies and prioritises traffic hop-by-hop across routed networks.