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Help with VLSM in regards to exam questions?

I need help figuring out the method to find the subnet ranges that are actually being used when they give you an IP somewhere within a VLSM network....

Like if they asked what possible IPs could be in the broadcast domain of this host 192.168.1.58/29 - Like I know there is 6 hosts but how do I know what the range is? I know if all the other subnets were also /29s the range would be 57-62.. But what do I do when they're different? Not sure how to work through it properly.. If I did a shitty job explaining my conundrum here's a sample from my practice test -

https://i.imgur.com/j21dHWD.png

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level 1
CCNP|CCDP|CCNA-V|CMNA2 points · 6 months ago

It sounds like you're answering your own question but maybe not realizing it, or you're confusing yourself.

Here you're asking how you'd know what the range is:

if they asked what possible IPs could be in the broadcast domain of this host 192.168.1.58/29 - I know there is 6 hosts but how do I know what the range is?

But then you provide the range here:

I know if all the other subnets were also /29s the range would be 57-62.

In a question like 'what IP can you use here without overlapping', you basically need to find all the ranges current used in the topology, and find which answer doesn't fall into any of the existing ranges.

In your example:

A covers .0 - .63
C covers .96 - .111
D covers .112 to .119

Which means options 1, 2, 3, and 5 would overlap, leaving option 4 (10.1.40.67 /27 covers .64 to .95, no overlap)

level 1
CCNA R&S / Sec - Cyber in progress2 points · 6 months ago · edited 6 months ago

-- another edit --

network id would actually be 192.168.1.56 or 192.168.1.64

-- end edit --

/29 means there are 8 ip addresses

192.168.1.58 is the network id .... it counts as 0, not usable.

.59 = 1

.60 = 2

.61 = 3

ect....

192.168.1.65 is the broadcast ip address, it counts as 7 and is not usable (0-7 = 8 numbers)

it would be useful to note that 192.168.1.60/29 is invalid and would never be in a routing table .... but it can be used as shorthand to indicate that the ip address belongs to an interface and it uses a /29 subnet mask

level 1
Now with more Cisco!1 point · 6 months ago

Like if they asked what possible IPs could be in the broadcast domain of this host 192.168.1.58/29 - Like I know there is 6 hosts but how do I know what the range is? I know if all the other subnets were also /29s the range would be 57-62

Here is where you are getting confused. There is only one range: 57-62.

If a subnet has 8 addresses (6 hosts), the starting address will always be a multiple of 8. And the ending address will always be a multiple of 8 minus 1. Always. Given that rule, there is only one range that meets that criteria, includes 8 addresses, and includes the 192.168.1.58 address: 192.168.1.56-63

You don't get to shift your range arbitrarily. This is because, in a /29, the first 29 bits of all addresses must be the same. 192.168.1.58 is 1100 0000 1010 1000 0000 0001 0011 1010 in binary. If the first 29 bits must stay the same, the lowest and highest number we can get is:

1100 0000 1010 1000 0000 0001 0011 1000 = 192.168.1.56 (64+16+8 = 56)

1100 0000 1010 1000 0000 0001 0011 1111 = 192.168.1.63 (56 + 4 + 2 + 1 = 63)

There aren't any other ranges that make sense.

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