Addressing and Subnet Formats
This domain tests IPv4 and IPv6 addressing, subnet masks and CIDR notation, public versus private ranges, and MAC address structure. You need to calculate usable host ranges quickly and recognize when an address belongs to the wrong subnet.
IPv4 rules
An IPv4 address is 32 bits in dotted decimal. The mask separates network and host bits; CIDR writes the prefix length, such as /24. Private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Loopback is 127.0.0.0/8. APIPA is 169.254.0.0/16. The all-zero network address and all-ones broadcast address cannot be assigned to hosts. A host's default gateway must be in the same subnet.
Subnetting worked example
Take 192.168.10.0/26. The mask is 255.255.255.192 and the block size is 64, so subnets begin at .0, .64, .128, and .192. Each has 62 usable hosts. For the .64 subnet: network 192.168.10.64, first host .65, last host .126, broadcast .127. A host at .130/26 is in the .128 subnet, not the .64 subnet. A host at .130/25 would be in 192.168.10.0/25, a different range again.
IPv6
IPv6 is 128 bits, written as eight hexadecimal groups. Leading zeros can be dropped, and one longest run of zero groups can be replaced by ::. A /64 is the standard subnet size. Global unicast is 2000::/3, link-local is fe80::/10, unique local is fc00::/7, multicast is ff00::/8, and loopback is ::1. EUI-64 builds an interface ID from a MAC address; SLAAC lets hosts self-configure. IPv6 has no broadcast.
MAC addresses
A MAC address is 48 bits written as 12 hex digits. The first 24 bits are the OUI; the last 24 bits identify the NIC. Broadcast is FF:FF:FF:FF:FF:FF. Switches learn source MAC addresses.
Traps
Do not assign network or broadcast addresses to hosts. Memorize block sizes: /24 is 256 addresses, /25 is 128, /26 is 64, /27 is 32. The :: shortcut may appear only once in an IPv6 address. Link-local addresses are not routable. Private addresses need NAT to reach the internet.