IP subnet-zero

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Apr 20th, 2010
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HI


I want to know if we can use the complete range of IP address in the ip subnet-zero e.g


IF the IP address range is 192.168.1.0 /28  and I have now around 16 networks


My question is can I use the complete IP address for Hosts in the network starting from 192.168.1.0 to 192.168.1.15


this will mean that I can use all the IP address starting from 192.168.1.0 to 192.168.1.255


waiting for a quick reply


K.Sinjish


Message was edited by: k_sinjish

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Jon Marshall Tue, 04/20/2010 - 07:49
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k_sinjish wrote:


HI


I want to know if we can use the complete range of IP address in the ip subnet-zero e.g


IF the IP address range is 192.168.1.0 /28  and I have now around 16 networks


My question is can I use the complete IP address for Hosts in the network starting from 192.168.1.0 to 192.168.1.15


this will mean that I can use all the IP address starting from 192.168.1.0 to 192.168.1.255


waiting for a quick reply


K.Sinjish


Yes you can because that's exactly what ip subnet-zero does.


Jon

k_sinjish Wed, 04/21/2010 - 00:46
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  Hi ..

Thanks for the update.

I am still bit dubious about this. Also please find the below example.

Eg:
Network ID :  192.168.1.1 /28

This  case   host  ip  address  are  from  192.168.1.0  to 15 , 16 to 31 , 32  to 47,….  .That means all the ips can be  assigned to hosts . ie no ips are leaving for  broadcast and network .

(Please correct me  if I am wrong ).

So my doubt is  for the  host  192.168.1.5 
1) What will be the broadcast cast ip ? and network address .  w
2) If it has  no  broad cast ip  , how can  a  subnet  do  broad casting  ?
3) If I am pinging to any broadcast ip  , will  any other pc respond apart from the broad  cast  ip   assigned host ?


Thanks & Regards ,
Sinjish.K

Jon Marshall Wed, 04/21/2010 - 00:59
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Network ID :  192.168.1.1 /28

This  case   host  ip  address  are  from  192.168.1.0  to 15 , 16 to 31 , 32  to 47,….  .That means all the ips can be  assigned to hosts . ie no ips are leaving for  broadcast and network .

(Please correct me  if I am wrong ).


You are wrong but i think i may have misunderstood your original question. ip subnet-zero does not allow you to use all the addresses within each subnet as host addresses. You still need a subnet address and a broadcast address. What ip subnet-zero does is simply allow you to use the subnet 192.168.1.0/28. Otherwise you would lose a whole subnet and have to start at 192.168.1.16/28.


192.168.1.0/28 means


subnet 192.168.1.0

available host addresses 192.168.1.1 -> 14

broadcast address 192.168.1.15


subnet 192.168.1.16/28 means


subnet 192.168.1.16

available host addresses 192.168.1.17 -> 30

broadcast address 192.168.1.31


etc..


so for 192.168.1.5 using a 192.168.1.0/28 subnet ->


subnet = 192.168.1.0

broadcast = 192.168.1.15


Jon

k_sinjish Wed, 04/21/2010 - 01:03
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Ok..So what are the  host ips  i am getting more by enabling the ip sub net zero


(In the above example )


Regards ,

sinjish.K

Jon Marshall Wed, 04/21/2010 - 01:05
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k_sinjish wrote:


Ok..So what are the  host ips  i am getting more by enabling the ip sub net zero


(In the above example )


Regards ,

sinjish.K


192.168.1.1 -> 192.168.1.14 with a broadcast of 192.168.1.15


Jon


Please rate helpful posts.

Carles P R Wed, 04/21/2010 - 05:47
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As far as I've found working with cisco machines. When you make subnets from a Class A / B / C IP address, if you don't have the "ip subnet-zero" command, you can't use the first one:


192.168.0.0 --> 4 subnets:


192.168.0.0/26

192.168.0.64/26

192.168.0.128/26

192.168.0.192/26


Without the command you could use just the .64; .128; .192


With the command you can use: .0; .64; .128; .192


That's what I found, specially with old routers where this command wasn't introduced by default and when you entered an IP address it gave an error.


But the unusable addresses of classfull IP address are also applied in subnets:

- All the host bits to 0: network IP address.

- All the host bits to 1: broadcast IP address.


So in the example above, each subnetwork would have the capacity for 62 host and would be 8 unusable IP addresses while in the original IP address there would be 254 usable IP addresses and just two unusable.


Well, I don't know why I wrote the last part but if it's helpful to anyone, it's ok!


Best regards,

Carles

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