written Lab 3:1

[Stevep]

Answer to question 3 might be incorrect or I don''t understand it.
192.168.100.66/27 is .224 and 32 block size. That should put .66 in the .64 subnet with the highest valid host 94 and a broadcast address of 95.

[aguilera]

Well, it seems to me that you have a perfect understanding of block sizes.

A prefix of /27 does produce a block size of 32.

So that means we can refer to the chart below to find out which network the .66 ip address belongs to.

Network: .0
1st available IP: .1
Last available IP: .30
Broadcast: .31

Network: .32
1st available IP: .33
Last available IP: .62
Broadcast: .63

Network: .64
1st available IP: .65
Last available IP: .94
Broadcast: .95

Network: .96
1st available IP: .97
Last available IP: .126
Broadcast: .127

and so on....

Now here is the deal. Does .66 belong to the 2nd available network, or the 3rd available network?

Well, that depends. What? :confused:

The host IP .66 will belong to the 2nd available network if "Subnet Zero" is disabled.... but if "subnet zero" is enabled then it would belong to the 3rd available network.;)

ip subnet-zero enables the IOS to accept .0 subnets.

I logged into one or our 2800 series routers, disabled subnet zero and tried to configure an IP address belonging to a zero subnet and got error messages. Here they are:

Router(config)#no ip subnet-zero
Router(config)#int fa0/0
Router(config-if)#ip add 10.0.0.1 255.224.0.0
Bad mask /11 for address 10.0.0.1
Router(config-if)#ip add 172.16.1.0 255.255.224.0
Bad mask /19 for address 172.16.1.0
Router(config-if)#ip add 192.168.10.1 255.255.255.224
Bad mask /27 for address 192.168.10.1
Router(config-if)#exit

I then enabled ip subnet-zero and was able to configure each of the ip addresses that gave me errors before. Take a look at the picture I included to see the results.

Router(config)#ip subnet-zero
Router(config)#int fa0/0
Router(config-if)#ip add 10.0.0.1 255.224.0.0
Router(config-if)#ip add 172.16.1.0 255.255.224.0
Router(config-if)#ip add 192.168.10.1 255.255.255.224
Router(config-if)#end

-aguilera