11 Replies Latest reply: May 17, 2011 3:02 AM by Brian

Dear All,

We have beneath setup in our  500 branch offices.

Router --- L2 Switch --- 172.X.X.X single subnet.

In the proposed setup we are planning to add two more subnet with L3 switches for inter-vlan routering

In additional two subnet one subnet is having 1050 host and second subnet just 20 hosts.

Router --- L3 switch ----- 172.X.X.X (existing) and two 10.0.0.0 subnet.

10.0.0.0/13 with per subnet host 2046 .

Branch office 1 subnet 1 --- 10.1.0.0 ---- host range 10.1.0.1 --- 10.1.7.254

branch office  1 subnet 2 - 10.1.8.0 --- host range 10.1.8.1 --- 10.1.15.254.

as per our ip address calculation all location with two subnet will have standard 2046 host/subnet.

In our proposed setup can we used some other ip addressing scheme to reduce host in 2nd subnet to 20.

BR.,

• ###### 1. Re: IP Addressing.

Aloha CiscoCCIE,

You may want to check your math.  Your 10.0.0.0/13 gives you 32 subnets with 524286 hosts per subnet.

prefix bits = 13

subnet bits = 13 - 8 = 5

host bits = 32 - 13 = 19

number of subnets = 2^5 = 32

number of hosts/subnet = 2^19 - 2 = 524286

By "other IP addressing scheme"  , I believe you mean VLSM.  1050 hosts would require 11 bits for the host portion.  32 - 11 = 21 bits for the network or 8 for the network and 13 for the subnet.

So you would want to use 10.0.0.0/21 for the subnet with 1050 hosts and 10.0.32.0/27 for the subnet with 20 hosts.  Or better yet use a different class for the subnet with 20 hosts such as 192.168.1.0/27.

10.0.0.0/21

prefix bits = 21

subnet bits = 21 - 8 = 13

host bits = 32 - 21 = 11

number of subnets = 2^13 = 8192

number of hosts/subnet = 2^11 - 2 = 2046

192.168.1.0/27

prefix bits = 27

subnet bits = 27 - 24 = 3

host bits = 32 - 27 = 5

number of subnets = 2^3 = 8

number of hosts/subnet = 2^5 -2 = 30

HTH

• ###### 2. Re: IP Addressing.

Hi Brain,

we have total 500 locations.

172.X.X.X and 192.X.X.X is alreay in use so we can't repeat it.

Per location we have two subnet

first subnet you calculation is corret.

10.0.0.0/21

prefix bits = 21

subnet bits = 21 - 8 = 13

host bits = 32 - 21 = 11

number of subnets = 2^13 = 8192

number of hosts/subnet = 2^11 - 2 = 2046

What about the second subnet 20 host requirement ?

BR.,

• ###### 3. Re: IP Addressing.

Based on the first subnet of /21, where would the next subnet start?  You can use any space above this for the second subnet of 20 hosts.

HTH

• ###### 4. Re: IP Addressing.

Hi Brain,

Attached proposed design snap for your reference.

Please note that we have total 500 location . the current setup is with single 172.X.X.X subnet running. We are adding two more vlan (subnet) in each location.

We have l3 switch for inter-vlan routing.

one subnet will have 1050 host and second subnet should accomadate 20 hosts.

Already 192.X.X.X and 172.X.X.X subnet is in use.

BR.,

• ###### 5. Re: IP Addressing.

Ok..can I ask why such a very large subnet and then a very small subnet.  What kind of LAN is this with 1050 hosts?  are these all PCs? Or a combination of PCs, servers, WAP, and IP phones?  Just curious.  If I had a LAN that big I would consider possible segmentation to reduce the size.  Another words create 4 LANs with 250+ hosts each.  That way each LAN can be its own /24.

Try using a spreadsheet to aid in creating your IP addressing plan.  You can use the "auto" fill feature and makes things much easier.

You should think about installing more than just one L3 switch to support 1050+ hosts.  what kind of site is this, again just curious to understand what could possibly be so big.

Brian

• ###### 6. Re: IP Addressing.

Hi Brain,

This is a school setup. All student will have laptop . In total we have average 1050 student and approx. 20 teacher.

10.0.0.0/21 and 10.0.0.0/27 will work.

BR.,

• ###### 7. Re: IP Addressing.

You have 1050 students and 20 teachers for each school and 500 schools?  Where in the world are you?  man that is big.

Are all students in the smae class?  You might want to think about maybe segmenting the 1050 students by grade.  You can have the teachers PC in the same subnet as the students  that way you do not have to single them out.

Based on the above numbers can I assume you have roughly 50 students to every 1 teacher?

No these two subnets 10.0.0.0/21 and 10.0.0.0/27 overlap.  With 1050 + 20 you would still only need a /21 for each location.  One possible solution might be as follows:

Site 1 - 10.1.0.0/21

Site 2 - 10.1.8.0/21

Site 3 - 10.1.16.0/21

Site 4 - 10.1.24.0/21

Site 5 - 10.1.32.0/21

etc......

You are going to need more than just one Layer 3 switch.  I would recommend at least two and at least one L2 switch per grade/class.

Best of luck.

Brian

• ###### 8. Re: IP Addressing.

Hi Brain,

we have complete 10.X.X.X to deploy in this 500 location.

Can we reserver 10.0.0.0/21  ex. 10.0.X.X to 10.150.X.X for students at each site.

Site 1 -  10.1.0.0/21

Site 2 - 10.1.8.0/21

Site 3 - 10.1.16.0/21

Site 4 - 10.1.24.0/21

Site 5 - 10.1.32.0/21

and for teacher's 10.151.0.0 /27 ex 10.151.X.X to 10.255.X.X at each locations.

Site 1 -  10.151.0.0/27

Site 2 - 10.151.0.32/27

Site 3 - 10.151.0.64/27

Site 4 - 10.151.0.96/27

Site 5 - 10.151.0.128/27

will it work?

BR.,

• ###### 9. Re: IP Addressing.

again I would not create a separate subnet for the teachers.  also, I would look at splitting up the 1050 students by grade or class.  You can use VACLs or private VLANs to keep the teacher for each class separate from the students.

Take each location separately.  you have 1050 students.  How many grades is this? How many classes in each grade?  How many students per class?

while you will route a /21 from HQ to a particular school, you will want to segment at the school based on grade/class etc.

Brian

• ###### 10. Re: IP Addressing.

Hi Brain,

We have lot of department in our ogranisation.

Our department is taking care of deploying ip's and core routing .

As i mention we got this requirement one location two subnet  1 - 1050 host and 2 - 20 host.

We don't have access to anything in schools or other department is not ready to release any information.

They just need an excel with ip addressing scheme.

BR.,

• ###### 11. Re: IP Addressing.

A single subnet of 1000+ hosts is not a very good idea.  LAN performance is drastically reduced when you start putting more than 250 hosts connections in a single LAN segment.

You are allowed to ask questions when designing your network.  They do not want to replease information, wow.  How do they expect you to design a proper network if you cannot ask questions.  You have your hands full.  Best of luck

Brian