But and there’s always a but . . . you can never use the first and the last IP address for

Understanding Internet Protocol | 81 As you can see, there are 16 values in each subnet host range, but you can’t use the first and last because they are all 0s and all 1s, respectively. So for example, in Subnet ID 1, the 16 and the 31 are unavailable; 16 is the actual subnet ID, and 31 is the broadcast address for that subnet. The usable IP addresses in that subnet are 17–30. In subnet ID 2, 32 and 47 are unavailable; therefore, the usable range is 33–46. Keep in mind that comput- ers in different subnets cannot communicate with each other by default. So, the IP address 192.168.50.17 cannot communicate with 192.168.50.3,3 and vice versa. Another item of note is that most operating systems including Windows either discourage, or flat out don’t allow, use of the first and last subnet IDs. This is to avoid confusion with the main network number prior to subnetting and the broadcasting segment. That was a lot of information. So, the best way to really explain the subnetting process is to do it. SUBNET A NETWORK GET READY. Let’s create a working subnet. Use the following information to create your working subnetwork: • Network: 192.168.50.0 • Subnet mask: 255.255.255.240 • Subnet ID to be used: ID 7 1. Go to the first computer we will call this computer1.

2. Disable any secondary network adapters. Make sure only one adapter is enabled; this

is the one you will use for the exercise.

3. Access the IP Properties window of computer1 and change the IP settings to reflect

the supplied subnet information. If you look back at Table 4-5, you will notice that subnet ID 7 dictates that you can use IP addresses between 192.168.50.112 and 192.168.50.127. However, remember the “golden rule”: You cannot use the first and last addresses. This means you are left with just 113–126. You can use any of these IPs as long as no two computers get the same IP address. For the purposes of sim- plicity, we chose the first valid IP for computer1, as shown in Figure 4-8. No gateway address or subnet mask is necessary. Be sure to write this out on paper as you go through the exercise. TAKE NOTE Figure 4-8 IP properties of computer1

4. Click OK for both windows.

5. Go to a second computer; we will call this computer2.

82 | Lesson 4 6. Disable any secondary network adapters. Make sure only one adapter is enabled; this is the one you will use for the exercise.

7. Access the IP Properties window of computer2 and change the IP settings to reflect

the supplied subnet information. This time select 192.168.50.114. Again, no gateway address or subnet mask is necessary.

8. Click OK for both windows.

9. Return to computer1 and open the command prompt.

10. Type ipconfigall and verify that your settings are as they should be.

11. Now type ping 192.168.50.114. You should get replies If not, recheck your configu-

ration on both computers.

12. Now try pinging a host that is not within your network, such as 192.168.1.1. Type

ping 192.168.1.1. It should not reply, and you should get either a transmit failed error or a message similar to “Destination host unreachable,” depending on the OS used. Either way, the connection will fail because it is on a different network number. Even if a device does exist on that network number, it will not reply to you.

13. Now try pinging a host that is not within your subnet, such as 192.168.50.17. Type

ping 192.168.50.17. It should not reply, and you should get a similar error message as in step 12. This is shown in Figure 4-9. This ping attempt failed because the host is on a different subnet and, by default, cannot communicate with computers on your subnet. Figure 4-9 Failed ping from a computer on a subnet You now have a working subnet that compartmentalizes the two computers from the other subnets on the network. Network engineers create subnets to compartmentalize networks. This could be to decrease broadcasts, increase data throughput, add security, limit access, and use IP addresses more wisely. There are many other examples of subnetting, and there are other kinds of subnet masks you can use beyond subnet mask 255.255.255.240. For example, 255.255.255.224 gives you the ability to have eight subnets recommended six usable and thirty usable IP addresses per subnet. You can also create subnets within Class A networks and Class B networks as well. Tables 4-6 through 4-8 show all of the possibilities when it comes to subnetting within any of the three IP classes. These tables take into account the fact that most OS and IOS internetwork operating system manufacturers will recommend not using the first or last subnet for any given subnetting scheme.