ANDing is the bitwise operation used to combine two equal-length binary values so that a bit is 1 only when both inputs have a 1 in that position. The truth table is simple: 1 AND 1 = 1, 1 AND 0 = 0, 0 AND 1 = 0, 0 AND 0 = 0. In networking it appears when an IPv4 address is masked to find the network ID. You convert the address and the subnet mask to binary, line up the bits, and apply the rule to each pair. Example: 192.168.1.130 AND 255.255.255.0 gives 192.168.1.0, which is the /24 network. With a /25 mask of 255.255.255.128 the same host falls in 192.168.1.128, because the most significant host bit is cleared by the mask. Routers use this operation at high speed to decide if a destination lies within a route. Hosts use it to decide whether to ARP or send to a gateway. Access lists and firewall rules also rely on bitwise checks, even when the interface shows dotted masks or CIDR prefixes. Wildcard masks on some vendor platforms invert the idea but still depend on the same logic. ANDing supports route summarisation, since aligned prefixes keep shared 1 bits and drop the rest. The concept extends to IPv6 even though you normally write only a prefix length. Devices still compare the first n bits using the same rule. Common mistakes include mixing decimal thinking with binary rules, forgetting that only 1 and 1 produce 1, and assuming that ANDing changes the original value in memory, which it does not unless you explicitly store the result.
