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Is this address on my network? Find the block, then the street

Take the mask's last non-255 octet away from 256 and you get the block size; the network addresses in that octet are the multiples of the block size. A host's network is the largest multiple at or below its number. Two hosts with the same network address can talk directly; two with different ones need the gateway.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A desktop with 192.168.1.77 cannot reach the printer at 192.168.1.130, and both have mask 255.255.255.192. They are on the same switch and the same cable run. They are not on the same network: 77 lives on the .64 street and 130 lives on the .128 street, and the printer's traffic has to go through a gateway that nobody configured on it.

How to think about it

Block size = 256 − (the interesting mask octet). Write the multiples: for 64 they are 0, 64, 128, 192. Find the multiple at or below each host's number; that is its network. The broadcast is the next multiple minus one. Same network address on both: direct. Different: through the gateway, and if there is no gateway on one of them, that is the fault.

Worked example

Mask 255.255.255.192 → block = 256 − 192 = 64
Networks start at 0, 64, 128, 192 in the last octet.
192.168.1.77 → 64 ≤ 77 < 128 → network 192.168.1.64, range .64 to .127
Seventy-seven sits in the second block. Its broadcast is .127.
192.168.1.130 → 128 ≤ 130 < 192 → network 192.168.1.128
The printer is in the third block.
.64 ≠ .128 → different networks; traffic goes via the gateway
Same switch, different streets. The fix is a gateway on the printer, or a printer address in the .64 block.

Your turn

Mask 255.255.255.192 again. Write the network address of 192.168.1.200.

192.168.1.200 → network 192.168.1.

The trap

Deciding by the first three numbers. With a /26, four different networks share 192.168.1; the last octet is where the decision is, and the mask is what tells you that.

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