Calculating The Mask For Ip Access Control - ADDER AdderView CATxIP 1000 User Manual

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
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
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..
procedure.should.this.occur..
explanation.for.
The.permissible.mask.values.(for.all.octets).are.as.follows:.
Mask octet Binary Number of addresses encompassed
255 11111111 1 address
254 11111110. 2 addresses.
252 11111100. 4 addresses
248 11111000. 8 addresses
240 11110000. 16 addresses
224 11100000.
32 addresses
192 11000000.
64 addresses
128 10000000. 128 addresses
0 00000000. 256 addresses
If.the.access.control.range.that.you.need.to.define.is.not.possible.using.one.
address.and.one.mask,.then.you.could.break.it.down.into.two.or.more.entries..
Each.of.these.entries.could.then.use.smaller.ranges.(of.differing.sizes).that,.
when.combined.with.the.other.entries,.cover.the.range.that.you.require.
For.instance,.to.accurately.encompass.the.range.in.the.earlier.example:
192.168.142.67 to 192.168.142.93.
You.would.need.to.define.the.following.six.address.and.mask.combinations.in.
the.IP.access.control.section:
Network/address entry Mask entry
192.168.142.67 255.255.255.255
192.168.142.68 255.255.255.252
192.168.142.72 255.255.255.248
192.168.142.80 255.255.255.248
192.168.142.88 255.255.255.252
192.168.142.93 255.255.255.255
defines 1 address (.67)
defines 4 addresses (.68 to .71)
defines 8 addresses (.72 to .79)
defines 8 addresses (.80 to .87)
defines 4 addresses (.88 to .92)
defines 1 address (.93)