VLSM Issue

[Melove]

Hi,

I have Cisco 3750X Switch, recently we try to advertise VLSM of 10.6.206.144\27 to the switch with OSPF Protocol.

Ip Pool we get after vlsm : 10.6.206.144-10.6.206.175

OSPF Advertisement are: Router ospf 10 

                                        Network 10.6.206.144 0.0.0.31 area 0

but after that in show running command it's become: network 10.6.206.128 0.0.0.31 area 0

we are unable to ping IPs 10.6.206.160-175. It's return with TTL Expired in Transit error.

Thanks in advanced.

[Giuseppe Larosa · Accepted solution]

Hello @Melove ,

your pool of addresses do not fit in a single /27 subnet.

network 10.6.206.128 0.0.0.31 area 0

the possibile subnets are aligned to 27th bit  and they are:

10.6.206.0/27

10.6.206.32/27

10.6.206.64/27

10.6.206.96/27

10.6.206.128/27

10.6.206.160/27

the last addresses fall in the next /27 subnet 10.6.206.160/27 this is the reason why they are not reachable.

You need to use a /26 subnet mask on the interface to cover all the addresses

Hope to help

Giuseppe

[Melove]

Thanks for Answer. but i am still confused. 

I have pool of 10.6.206.0 and created 6 subnets: S1=32, S2=16, S3=16, S4=64, S5=16 and s6=32. 

and only facing issue in last subnet which pool size is 32 host. 

s5 end at 10.6.206.143 and s6 start with 10.6.206.144 to 10.6.206.175/27.

thanks in advanced.

[Giuseppe Larosa]

Hello @Melove ,

you have to review the way you perform subnetting.

144   is 10010000 in binary and it cannot be  a base address for a /27

100 || 10000    but the bits after the subnet boundary || should all be zero. And here there is one set

10.6.206.144 is a suitable base address for /28

1001|| 0000

Divide the network in /27 blocks  and then take one or more of them and split each of them in two to accomodate /28 subnets.

The /27 blocks can be used as they are where needed.

Hope to help

Giuseppe

[Joseph W. Doherty]

Ah, your misunderstand VLSM allocations.  VLSM blocks are always aligned on powers of 2.

Simple example, we want to allocate a /30 (4 addresses), a /29 (8 addresses) and another /30 (4 addresses).

So you think, starting with 10.0.1.0 I'll do:

/30 10.0.1.0..10.0.1.3

/29 10.0.1.4..10.0.1.11 (invalid)

/30 10.0.1.12..10.0.1.15

The second allocation is invalid.

There are two valid allocations:

/30 10.0.1.0..10.0.1.3

/30 10.0.1.4..10.0.1.7

/29 10.0.1.8..10.0.1.15

or

/29 10.0.1.0..10.0.1.7

/30 10.0.1.8..10.0.1.11 

/30 10.0.1.12..10.0.1.15

The above doesn't explain why the first /29 is invalid while the other two /29s are valid, for that you need to understand VLSM, which @Giuseppe Larosa tries to explain.

Also note, if you use a /29 with any host value 10.0.1.0..10.0.1.7 it will assume it's in that /29 and similar with 10.0.1.8..10.0.1.15.  This is why you're surprised your host and /# are NOT in the block you expect, in my example, that would be hosts in the range 10.0.1.4..10.0.1.11, which actually is the top half of one /29 and the bottom half of another /29.

[Martin L · Accepted solution]

created 6 subnets: S1=32, S2=16, S3=16, S4=64, S5=16 and s6=32. Not sure what those numbers after "=" indicate (mean)?

You can find VLSM explained and examples in youtube. Here is my brief summary; You use and think in term of blocks (128, 64,32,16,etc)- see Magic Table.  Smaller blocks will fit into larger one.  you must start with the largest host number requirement and assign block to it, i.e. I need 60 hosts before 30 hosts. this is because /27 fits into /26; U used 1 /26 block and have 3 left.  you can fit multiple smaller blocks into larger one.  if you do not start that way, it will be a bit harder to figure out your block range and you will waste IPs but it is not total collapse. Note that block 32, aka /27, gives u 30 hosts and /64 block, aka /26, comes with 62 hosts. Even if u do not need 30, you must follow block range and some IPs may be left unused within the block range.  You cannot go over the block range (boundary).

Best ways is to use Magic table (subnetting magic table) which is decimal way of subnetting (not binary way). Attach is my old doc with Magic table which u must memorize for exam (and u can draw it before exam on piece of sheet provided) . See attach

Regards, ML
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[Melove]

Hi,

After two days work i finally understand the bugs in my VLSM. I found that my subnets are overlapping with each other(in simple terms). And Thanks all of you for the help.