Subnetting Lab 5 EMS Simplified

Read the lab details listed below, and then answer the questions listed in the lab exercise.

 “EMS” Simplified Method for CIDR Class B

In subnetting CIDR “Classless Inter-Domain Routing” for a Class B address the process is again very much the same in creating the cheat sheet chart that we did in a Class C address. The difference now is that now we are using the third and fourth octets that we used in the Class C address. This is because the first two octets are the network values, and the last two Octets are the host values as shown here: 

172.16. 3 . 5  

              Network  Host  address.

The additional difference is that in this example we use a Class-C mask on this Class-B

Now we will take the same standard Class B guide, but we want a value in the 4^th octet. For example, we again want 30 host IP’s but we want them on 2000 networks. Notice below that we get this from the guide at the 32-bit value in the 4^th octet as identified below.

Below we see the Class B Subnetting Guide created the same way we did the Class C Subnetting Guide.

Class-B Subnetting Guide

    Groups    128      64        32        16        8         4        2        1

1. IP’s        32768  16384  8192  4096   2048  1024   512   256   128     64      32     |   16        8           4           2         1

2. Hosts    32766  16382  8190  4094   2046  1022   510   254   126     62      30     |   14        6           2           0          

3. Subnets     0         2          6         14        30      62     126   254   510   1022  2046  | 4094   8190   16382   32766

Remember that using this Cheat-Sheet Guide gives us the usable numbers available for the creation of subnetworks and hosts. The numbers on line 2 are again the numbers of usable hosts. The numbers on line 3 are again the number of usable Sub-networks. That is because we took away both the Network IP and Broadcast IP from each out of line 1.

We again subnet a Class B with this guide by giving up bits from left to right starting with the 128 bit on the far left in the 3^rd octet and moving right to get the subnet range we want. If you want 30 host computers on each of the 2000 subnetworks You would give up the 128 bit through the 1 bit in the 3^rd octet as well as the 128, the 64, and the 32 bit values of line 1 of the 4^th octet. That means you would borrow all 8 bits from the 3^rd octet and three more bits from the 4^th octet. This now moves down into a Class C range, hence the term CIDR, which stands for Classless Inter-Domain Routing. You can identify this by drawing a line down from the 4 past the 1022 to the 62 as indicated in the example above.

Again, we started with the default mask of 255.255.0.0 To get the new subnet mask we add the values of each all of the borrowed bits from left to right in the 3^rd octet and those three bits of the 4^th octet on line 1. If you add all the bits in the 3^rd octet, you would get 255; then you would add the 128 and 64 and the 32 bits from the 4^th octet together and get 224. The new subnet mask is 255.255.255.224

These same processes for a CIDR Class-B can be seen here in Table 3.

Table 3

Subnet 1	3rd Octet 0.	172.16.0	4th Octet Values 0---------31	Assignable 4th Octet Values 1-------30
Subnet 2	0.	172.16.0	32--------63	33------62
Subnet 3	0.	172.16.0	64--------95	65------94
Subnet 4	0.	172.16.0	96-------127	97-----126
Subnet 5	0.	172.16.0	128------159	129----158
Subnet 6	0.	172.16.0	160------191	161----190
Subnet 7	0.	172.16.0	192------223	193----222
Subnet8	0.	172.16.0	224------255	225----254
Subnet 9	1.	172.16.1	0---------31	1-------30
Subnet 10	1.	172.16.1	32--------63	33------62
Subnet 11	1.	172.16.1	64--------95	65------94
Subnet 12	1.	172.16.1	96-------127	97-----126
Subnet 13	1.	172.16.1	128------159	129----158
Subnet 14	1.	172.16.1	160------191	161----190
Subnet 15	1.	172.16.1	192------223	193----222
Subnet16	1.	172.16.1	224------255	225----254

This table would continue down until the 3rd octet has used all values from 0 to 255.

Subnetting Lab 5 Exercise

Using the Class B Subnetting Guide, answer the following: