What is the lowest prefix possible and the lowest IP@

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Feb 27th, 2009
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1) The lowest prefix possible is /8 ? Or we can go further down ?

For instance could we summarize:

8.0.0.0/8

9.0.0.0/8

into

8.0.0.0/7


2) The lowest IP address that we could assign to a host is 0.0.0.1 ?

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Edison Ortiz Fri, 02/27/2009 - 10:20
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1) Technically, you can go as low as 8.0.0.0/1 which provides a host range from 0.0.0.1 to 127.255.255.255.254


2) That's a good question. While the summarization allows you to include such host, you are unable to enter such IP in Windows or Cisco IOS


Rack1R1(config)#int f0/1

Rack1R1(config-if)#ip add 0.0.0.1 255.255.255.0

Not a valid host address - 0.0.0.1

Rack1R1(config-if)#ip add 0.0.0.2 255.255.255.0

Not a valid host address - 0.0.0.2

Rack1R1(config-if)#ip add 1.0.0.1 255.255.255.0


HTH,


__


Edison

badalam_nt Fri, 02/27/2009 - 11:40
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Thanks Edison.

Do you have any idea then what is the range 0.0.0.1 to 0.255.255.254 used for, as long as we are not allowed to assign any of those IP addresses to any host nor to any router interface ?

Edison Ortiz Fri, 02/27/2009 - 11:58
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Do you have any idea then what is the range 0.0.0.1 to 0.255.255.254 used for


AFAIK, it isn't used for anything at the moment.


It just happens that range will be included in you were to summarize 8.0.0.0/1


HTH,


__


Edison.

bs6825 Fri, 02/27/2009 - 15:45
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Hate to disagree with a fellow CCIE, but an 8.0.0.0/1 would be invalid. a /1 would seem to indicate that the first bit only (the 128 bit) would be the network address. RFC specifies that a network address cannot be all 0's or 1's, that is why 127.x.x.x/8 is not valid and no 0.x.x.x/8 or less is not valid.


I have not tried this, but I would guess that a /7 to summarize 8.0.0.0 and 9.0.0.0 (0000 1000 and 0000 1001 summarized to 0000 100) would be valid.

Edison Ortiz Fri, 02/27/2009 - 16:28
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Hate to disagree with a fellow CCIE, but an 8.0.0.0/1 would be invalid. a /1 would seem to indicate that the first bit only (the 128 bit) would be the network address.


It's not invalid. The router will interpret this summary as 0.0.0.0 128.0.0.0


Rack1R1(config)#router bgp 1

Rack1R1(config-router)#aggre

Rack1R1(config-router)#aggregate-address 8.0.0.0 128.0.0.0

Rack1R1(config-router)#do show run | be router bgp 1

router bgp 1

no synchronization

bgp log-neighbor-changes

aggregate-address 0.0.0.0 128.0.0.0


While /7 will just include the two major subnets of 8.0.0.0 and 9.0.0.0 - the OP wanted to know if he could use a lower value on the prefix.


I didn't say all 0s will be a network address, please read the entire thread.


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