Adder Technology EPS-M8 Manuel D’Utilisation

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.

.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

1110 0 0 0

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.....

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..