Adder Technology AdderView CATx EPS-S8 - Manual (Page 101)

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Appendix 6 – Addresses, masks and ports
IP.address,.network.masks.and.ports.are.all.closely.linked.in.the.quest.for.one.
device.to.find.another.across.disparate.network.links.
IP addresses
As.a.rough.analogy,.consider.how.you.use.the.telephone.system..The.phone.
number.for.Adder.Technology.in.the.UK.is.
0044 (0)1954 780044..This.number.
consists.of.three.distinct.parts:
•. 0044.connects.from.another.country.to.the.UK
•. (0)1954.selects.the.main.telephone.exchange.in.the.Bar.Hill.area.of.
Cambridgeshire,.and
•. 780044.is.the.unique.code.for.Adder.Technology.within.Bar.Hill.
The.important.parts.of.the.whole.number.depend.on.where.you.are..If.you.were.
based.in.the.same.local.area.as.Adder.Technology,.there.would.be.no.point.
in.dialling.out.of.the.UK,.or.even.out.of.the.area..The.only.part.of.the.whole.
number.that.you.are.interested.in.is.the.final.part:.780044.
In.a.similar.way.to.the.various.parts.of.the.telephone.number,.the.four.sections.
(or.Octets).of.every.IP.address.have.different.meanings.or.“weights”..Consider.
the.following.typical.IP.address:.
192.168.142.154
192.is.the.most.global.part.of.the.number.(akin.to.the.0044.of.the.phone.
number).and.
154.is.the.most.local.(similar.to.the.780044.unique.local.code.of.
the.phone.number)...
When.two.network.devices.communicate.with.each.other,.they.always.“dial.the.
whole.number”.regardless.of.their.respective.locations.in.a.network..However,.
they.still.need.to.know.whether.the.other.device.is.local.to.them.or.not,.and.this.
is.where.the.net.mask.comes.into.play.
Net masks
The.net.mask.(or.sub-net.mask).informs.a.device.as.to.its.own.position.within.a.
network..From.this.it.can.determine.whether.any.other.device.is.within.the.same.
local.network.or.is.situated.further.afield.
Taking.the.telephone.number.analogy.given.in.the.IP.address.section,.in.order.
to.use.the.telephone.system.efficiently,.it.is.vital.for.you.to.know.your.location.
relative.to.the.person.you.are.calling..In.this.way.you.avoid.dialling.unnecessary.
numbers...
When.one.network.device.needs.to.talk.to.another,.the.first.thing.that.it.will.
do.is.a.quick.calculation.using.its.own.IP.address,.the.other.device’s.IP.address.
and.its.own.net.mask..Suppose.a.device.with.address.
192.168.142.154.and.
net.mask. 255.255.255.0 needed.to.communicate.with.a.device.at.address.
192.168.142.22..The.sending.device.would.perform.several.calculations:
The.reason.for.doing.this?.It.makes.the.network,.as.a.whole,.much.more.
efficient..If.every.message.for.every.recipient.was.shoved.straight.out.onto.the.
Internet,.the.whole.thing.would.grind.to.a.halt.within.seconds..Net.masks.keep.
local.traffic.just.that.-.local.
Want to know more?
1 .The.net.mask.is.used.to.determine.the.local.and.global.parts.of.the.sender’s.IP.
address..Where.there.is.255.in.the.mask,.the.corresponding.address.slips.through,.
where.there.is.a.0,.it.is.blocked.
2 .Where.the.net.mask.was.0,.the.corresponding.part.of.the.result.is.also.zero.-.this.
section.is.now.known.to.be.the.local.part.of.the.IP.address.
3 .The.same.process.is.carried.out.for.the.destination.address,.again.using.the.sender’s.
net.mask..Now.the.local.parts.of.both.addresses.have.been.equalised.to.zero,.because.
their.values.are.not.important.in.determining.whether.they.are.both.in.the.same.local.
network....
4 .The.results.of.the.two.net.mask.operations.are.now.compared,.if.they.match,.the.
destination.is.local..If.not,.then.the.sender.will.still.use.the.same.full.destination.IP.
address.but.will.also.flag.the.message.to.go.via.the.local.network.gateway.and.out.
into.the.wider.world..
Sending.
device.IP.
address
Net.mask
Destination.
device.IP.
address
192.168.142.000. .=.. 192.168.142.000 .?...Answer:. Yes
Address
is local
1
2
4
192
192
168
168
142
142
154
000
192
192
168
168
142
142
22
000 Result
3