Instruction 02-ip-addresses-subnets-gateways-and-routing: Reading

An IPv4 address is a 32-bit number. CIDR notation combines that address with a prefix length, such as 192.168.50.77/26. The first 26 bits identify the network and the remaining 6 bits identify positions within that network. A /26 therefore contains 2^6, or 64, total addresses. For an ordinary broadcast subnet, the all-zero host value is the network address and the all-one host value is the broadcast address, leaving 62 conventional host addresses. The four /26 boundaries inside 192.168.50.0/24 begin at .0, .64, .128, and .192. Consequently, 192.168.50.77/26 belongs to 192.168.50.64/26, has broadcast address 192.168.50.127, and has the conventional host range 192.168.50.65 through 192.168.50.126.

A subnet answers the local-delivery question: can the destination be reached directly on the local link, or must the packet be sent to a router? The sender compares the destination against its connected prefixes. If the destination is on-link, the sender resolves the destination's link-layer address and sends directly. If it is off-link, the sender searches its routing table. A default gateway is not automatically used for every packet; it is the next hop associated with the default route and is selected only when no more-specific route wins. The gateway must normally have an address reachable through a connected route, because the host needs a way to deliver the frame to that router before the router can forward the packet.

Routing follows longest-prefix matching. Suppose a host has routes for 10.0.0.0/8, 10.20.0.0/16, and 0.0.0.0/0. A packet for 10.20.30.40 matches all three, but /16 is the longest prefix and wins. A packet for 10.30.1.1 matches /8 and /0, so /8 wins. A packet for 203.0.113.20 matches only /0 and follows the default route. Metrics are usually consulted after prefix specificity and should not be treated as a way for a low-metric default route to override a more-specific route.

Routes may be connected, manually configured, learned through a routing protocol, or installed by network-management software. A host commonly has a connected LAN route, routes for local or special-purpose destinations, and a default route through the LAN router. A router has multiple interfaces and forwards traffic between prefixes according to its routing table and policy. Routing and address translation are separate concepts: routing chooses a path, while address translation may rewrite packet addresses at a boundary. The lab is deliberately observational and simulated. It reads current host state and performs calculations offline, but it does not add addresses, replace routes, restart network services, or affect connectivity.