Addresses & Masks

Network Is Address AND Mask

The network address is not guessed from the dotted text. It is the exact bitwise AND of the host address and its prefix mask.

highlighted = computed this step

Why AND finds the network

The mask is useful because every address bit can be combined with the matching mask bit. A one keeps the address bit; a zero clears it.

mask prefix=/26\text{mask prefix}=/26

Network is address AND mask

192.168.1.130 AND /26 keeps the prefix bits and zeros the host bits. The recomputed network is 192.168.1.128.

0xc0a80182&0xffffffc0=0xc0a801800xc0a80182\mathbin{\&}0xffffffc0=0xc0a80180
Address AND maskThe network row is recomputed as address bits AND mask bits.Address AND mask/26 mask 255.255.255.192address11000000101010000000000110000010192.168.1.130 0xc0a80182mask11111111111111111111111111000000255.255.255.192 0xffffffc0network11000000101010000000000110000000192.168.1.128 0xc0a80180

The table is checked

The author supplies the network row, but the engine recomputes every bit from the address and mask. If the displayed row disagrees, the diagram is rejected.

network=0xc0a80180\text{network}=0xc0a80180
Address AND maskThe network row is recomputed as address bits AND mask bits.Address AND mask/26 mask 255.255.255.192address11000000101010000000000110000010192.168.1.130 0xc0a80182mask11111111111111111111111111000000255.255.255.192 0xffffffc0network11000000101010000000000110000000192.168.1.128 0xc0a80180

Summary

Network arithmetic is bitwise AND: mask ones preserve network bits, mask zeros clear host bits. Address arithmetic only; timing/throughput is not modeled here.

network=0xc0a80180\text{network}=0xc0a80180
Address AND maskThe network row is recomputed as address bits AND mask bits.Address AND mask/26 mask 255.255.255.192address11000000101010000000000110000010192.168.1.130 0xc0a80182mask11111111111111111111111111000000255.255.255.192 0xffffffc0network11000000101010000000000110000000192.168.1.128 0xc0a80180