Special IPv4 Addresses

Type Range / rule Routable Network address All host bits = 0 (192.168.1.0/24) No — never a host Broadcast All host bits = 1 (192.168.1.255) No — never a host Loopback 127.0.0.0/8 (usually 127.0.0.1) No — router loopbacks make great Router IDs APIPA 169.254.0.0/16 No — Windows self-assigned when DHCP fails; no default gateway Exam […]

Private vs Public IP Addresses

Private addresses are used internally and cannot be routed on the internet; public addresses are globally routable (assigned by ISP/RIR). RFC 1918. Ranges 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 Motivation: IPv4 exhaustion, internal networks without global routing, isolation. Private space is reused everywhere; NAT translates private ↔ public at the edge. 10.x / 172.16–31.x / 192.168.x → always […]

The IPv4 Header

The IPv4 header is a variable Layer 3 header (RFC 791, 20–60 bytes) containing everything routers use to forward, fragment and deliver packets. Fields (exam critical) Field Size Purpose Version 4 bits Always 4 IHL 4 bits Header length in 32-bit words (5–15) DSCP / ECN 8 bits QoS prioritisation + congestion signal Total Length […]