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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? 00 address Net.mask address 3 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: 1681421541681421541441111000014410011010of.the.net.mask.and.look.at.the. 10010000Inside a bit-wise AND function When.you.“open.up”.the.last.octet. 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 192 Binary equivalent of 192 Binary octet after AND operation with net mask Decimal equivalent of 00 0000 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..... ... ........... .... ..... .. ... .... . ........ .... .... ... . .... 0 Calculating the mask for IP access control The.IP.access.control.function.uses.a.standard.IP.address.and.a.net.mask. notation.to.specify.both.single.locations.and.ranges.of.addresses..In.order.to. use.this.function.correctly,.you.need.to.calculate.the.mask.so.that.it.accurately. encompasses.the.required.address(es). Single locations Some.of.the.simplest.addresses.to.allow.or.deny.are.single.locations..In.this.case. you.enter.the.required.IP.address.into.the.‘Network/Address’.field.and.simply. enter.the.‘Mask’.as.255.255.255.255.(255 used throughout the mask means that every bit of the address will be compared and so there can only be one unique address to match the one stated in the ‘Network/Address’ field). All locations The.other.easy.setting.to.make.is.ALL.addresses,.using.the.mask.0.0.0.0..As. standard,.the.IP.access.control.section.includes.the.entry:.+0.0.0.0/0.0.0.0 The.purpose.of.this.entry.is.to.include.all.IP.addresses..It.is.possible.to.similarly. exclude.all.addresses,.however,.take.great.care.not.to.do.this.as.you.instantly. render.all.network.access.void..There.is.a.recovery procedure.should.this.occur.. Address ranges Although.you.can.define.ranges.of.addresses,.due.to.the.way.that.the.mask. operates,.there.are.certain.restrictions.on.the.particular.ranges.that.can.be.set.. For.any.given.address.you.can.encompass.neighbouring.addresses.in.blocks.of. either.2,.4,.8,.16,.32,.64,.128,.etc..and.these.must.fall.on.particular.boundaries.. For.instance,.if.you.wanted.to.define.the.local.address.range:. 192.168.142.67 to 192.168.142.93. The.closest.single.block.to.cover.the.range.would.be.the.32.addresses.from:. 192.168.142.64 to 192.168.142.95.. The.mask.needed.to.accomplish.this.would.be:.255.255.255.224.. When.you.look.at.the.mask.in.binary,.the.picture.becomes.a.little.clearer..The. above.mask.has.the.form:.11111111.11111111.11111111.11100000. Ignoring.the.initial.three.octets,.the.final.six.zeroes.of.the.mask.would.ensure. that.the.32.addresses.from..64.(01000000).to..95.(01011111).would.all.be. treated.in.the.same.manner..See.Net masks - the binary explanation.for. details........ When.defining.a.mask,.the.important.rule.to.remember.is: There must be no ‘ones’ to the right of a ‘zero’.. For.instance,.(ignoring.the.first.three.octets).you.could.not.use.a.mask.that.had. 11100110 because.this.would.affect.intermittent.addresses.within.a.range.in.an. impractical.manner..The.same.rule.applies.across.the.octets..For.example,.if.you. have.zeroes.in.the.third.octet,.then.all.of.the.fourth.octet.must.be.zeroes.. The.permissible.mask.values.(for.all.octets).are.as.follows:. Mask octet Binary Number of addresses en...

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