101
Net masks - the binary explanation
To.really.understand.the.operation.of.a.net.mask.it.is.necessary.to.delve.deeper.
into.the.life.blood.of.computers.–.binary;.this.is.native.digital,.where.everything.
is.either.a.1.(one).or.0.(zero),.on.or.off,.yes.or.no..
The.net.mask.operation.described.on.the.previous page.is.known.as.a.‘bit-wise.
AND.function’..The.example.of.255.255.255.0.is.handy.because.the.last.octet.
is.completely.zero.and.is.“clean”.for.illustrative.purposes..However,.actual.net.
mask.calculations.are.carried.out,.not.on.whole.decimal.numbers,.but.bit.by.bit.
on.binary.numbers,.hence.the.term.‘bit-wise’..In.a.real.local.network,.a.net.mask.
might.be.255.255.255.240..Such.an.example.would.no.longer.be.quite.so.clear,.
until.you.look.at.the.net.mask.in.its.binary.form:
.11111111.11111111.11111111.11110000
In.this.case,.the.four.zeroes.at.the.end.of.the.net.mask.indicate.that.the.local.
part.of.the.address.is.formed.by.only.the.last.four.bits..If.you.use.the.diagram.
from.the.previous.example.and.insert.the.new.net.mask,.it.will.have.the.
following.effect.on.the.final.result:
192
192
168
168
142
142
154 154
144
1
1
1
0
0
0
0
1
1
1
0
0
0
1
0
0
0
111
0
000
144
Thus,.when.154.is.bit-wise ANDed.with.240,.the.result.is.144..Likewise,.any.
local.address.from.192.168.142.144.through.to.192.168.142.159.would.
produce.exactly.the.same.result.when.combined.with.this.net.mask,.hence.they.
would.all.be.local.addresses..However,.any.difference.in.the.upper.three.octets.
or.the.upper.four.bits.of.the.last.octet.would.slip.through.the.mask.and.the.
address.would.be.flagged.as.not.being.local.....
Inside a bit-wise AND function
When.you.“open.up”.the.last.octet.
of.the.net.mask.and.look.at.the.
binary.inside,.you.can.see.the.last.
four.zero.bits.preventing.any.1’s.in.
the.address.from.falling.through..
Decimal octet prior to AND
operation with net mask
Binary equivalent of 154
Binary octet after AND
operation with net mask
Decimal equivalent of 10010000