The mask splits network from host

Last step: an IP address is just 32 bits. Now we lay a ruler over those same bits to mark where the network ends and the host begins.

3 quick questions · about 2 min · no sign-up

Question 1 of 3

The subnet mask is also 32 bits, laid over the same address. But its bits aren't random — they follow one strict shape. What is it?

• All 1s on the left, then all 0s — and the line never breaks
• It can be any mix of 1s and 0s, like 10110100...
• All 0s first, then all 1s
• I'm not sure

You said: All 1s on the left, then all 0s — and the line never breaks

Exactly. A mask is always a solid block of 1s followed by a solid block of 0s — like 11111111 11111111 11111111 00000000. The 1s sit on the left, the 0s on the right, and they never interleave.

You said: It can be any mix of 1s and 0s, like 10110100...

That's the trap — masks can't be a random scatter. A valid mask is always all 1s on the left, then all 0s on the right, with no breaks: like 11111111 11111111 11111111 00000000. The 1s must stay contiguous.

You said: All 0s first, then all 1s

Right idea — two solid blocks — but flipped. The 1s come FIRST, on the left, then the 0s: 11111111 11111111 11111111 00000000. The left side is always the 1s.

You said: I'm not sure

No worries. A mask is always a solid run of 1s on the left, then a solid run of 0s on the right — like 11111111 11111111 11111111 00000000. Never mixed, never broken.

Another way to see it

Picture it as a sliding wall: every bit to the left of the wall is a 1, every bit to the right is a 0. You can move the wall, but you can't punch holes in it — no stray 0 inside the 1s, no stray 1 inside the 0s.

Those two blocks aren't just decoration — each one labels a part of the address.

Question 2 of 3

So the mask sits on top of the address, bit for bit. What does a 1 in the mask mark in the address below it?

• That bit belongs to the NETWORK part
• That bit belongs to the HOST part
• That bit is switched on (set to 1) in the address
• I'm not sure

You said: That bit belongs to the NETWORK part

Yes. Where the mask shows a 1, that bit of the address is network. Where the mask shows a 0, that bit is host. The mask is literally a stencil: 1 = network, 0 = host.

You said: That bit belongs to the HOST part

Flipped. The 1s mark the NETWORK part; the 0s mark the host part. Mnemonic: a 1 stands tall like the fixed network, a 0 is the open space left for hosts.

You said: That bit is switched on (set to 1) in the address

Careful — the mask doesn't change the address's value. It only LABELS each position: a 1 in the mask says 'this bit is network,' a 0 says 'this bit is host.' It marks roles, not values.

You said: I'm not sure

All good. A 1 in the mask marks that bit as NETWORK; a 0 marks it as host. The mask is a stencil laid over the address: 1 = network, 0 = host.

Now the payoff: that slash notation you've seen, like /24, is just a shorthand for this.

Question 3 of 3

You're told a network is /24. Counting from the left of the 32 bits, how many bits are network and how many are host?

• 24 network bits, 8 host bits
• 24 host bits, 8 network bits
• 24 network bits, 24 host bits
• I'm not sure

You said: 24 network bits, 8 host bits

Exactly. /24 means 'the first 24 bits are 1s' — so 24 network bits, and the remaining 32 − 24 = 8 are 0s, the host bits. The slash number is just the count of leading 1s in the mask.

You said: 24 host bits, 8 network bits

Reversed. /24 counts the leading 1s, which are the NETWORK bits — so 24 network, and the leftover 32 − 24 = 8 are host. The slash always counts network bits first.

You said: 24 network bits, 24 host bits

Close on the network side, but the host count is off. An address is only 32 bits total, so 32 − 24 = 8 host bits, not 24. /24 means 24 ones, 8 zeros.

You said: I'm not sure

No problem. /24 means the first 24 bits are 1s — that's 24 network bits. The rest, 32 − 24 = 8, are 0s: the host bits.

The takeaway

The subnet mask is a 32-bit ruler laid over the address: a solid block of 1s (network) then a solid block of 0s (host). The slash, like /24, is just how many leading 1s there are — so /24 splits 32 bits into 24 network and 8 host.

Next step

You can see the mask draw the line between network and host bits. Next: the actual operation that applies the mask to an address to extract the network — bitwise AND.