Today’s post begins the discussion of some answers to the lab mentioned in the title. This post makes little sense without reading the lab post first! Take a look there, and take a crack at doing the lab, at least on paper, before reading below the fold here. Today, I’ll look at Part 1 in that lab, namely the IP addressing. Enjoy!
First, focus on the subnet IDs. The lab, in part 1, tells you the specific subnet IDs to use on the four LAN subnets, with more general instructions about the three WAN subnets. To get a handle on the details, just treat the problem as a general subnet design problem, with the required network 172.23.0.0, and with all subnets using the required /26 (both facts required per the lab instructions).
When using mask /26 (255.255.255.192), if you use the same magic number shortcut shown in my books, the magic number will be 64, and the interesting octet is the fourth octet. That is, all the subnet IDs are a multiple of 64 in the fourth octet. Since it’s a class B network that you’re subnetting, the subnet part exists in the 3rd and 4th octets, so counting sequentially, the subnet IDs will be:
The lab also dictates the IP addresses to use in each subnet. On the LANs, the lab says to use the numerically lowest numbers, so finding those is easy: just add 1 to the respective subnet IDs. However, the lab asks for the numerically highest IP address on the R2, R3, or R4 end of each WAN subnet, so that means you have to calculate the range of addresses in each WAN subnet.
You can use any method you like to find the range of addresses, but if you again are using the decimal method described in the books, in the interesting octet (4th octet in this case), add the magic number to the subnet ID, and subtract 1. That’s the broadcast address in the subnet. Subtract 1 more for the last usable address in the subnet. Table 2 lists the WAN subnet IDs and broadcast addresses so you can check your math.
From here, it’s just a matter of adding the configuration. Depending on where you are in your studies, this may or may not be a big deal. I’ll list the IP addressing config, and conveniently ignore the Frame Relay issues, deferring those to a later post.
First, focus on the subnet IDs. The lab, in part 1, tells you the specific subnet IDs to use on the four LAN subnets, with more general instructions about the three WAN subnets. To get a handle on the details, just treat the problem as a general subnet design problem, with the required network 172.23.0.0, and with all subnets using the required /26 (both facts required per the lab instructions).
When using mask /26 (255.255.255.192), if you use the same magic number shortcut shown in my books, the magic number will be 64, and the interesting octet is the fourth octet. That is, all the subnet IDs are a multiple of 64 in the fourth octet. Since it’s a class B network that you’re subnetting, the subnet part exists in the 3rd and 4th octets, so counting sequentially, the subnet IDs will be:
- 172.23.0.0
- 172.23.0.64
- 172.23.0.128
- 172.23.0.192
- 172.23.1.0
- 172.23.1.64
- 172.23.1.128
- 172.23.1.192
- 172.23.2.0
- 172.23.2.64
- …and so on
The lab also dictates the IP addresses to use in each subnet. On the LANs, the lab says to use the numerically lowest numbers, so finding those is easy: just add 1 to the respective subnet IDs. However, the lab asks for the numerically highest IP address on the R2, R3, or R4 end of each WAN subnet, so that means you have to calculate the range of addresses in each WAN subnet.
You can use any method you like to find the range of addresses, but if you again are using the decimal method described in the books, in the interesting octet (4th octet in this case), add the magic number to the subnet ID, and subtract 1. That’s the broadcast address in the subnet. Subtract 1 more for the last usable address in the subnet. Table 2 lists the WAN subnet IDs and broadcast addresses so you can check your math.
Table 2: WAN Subnet IDs and Subnet Broadcast Addresses
| Subnet ID | Subnet B’cast |
| 172.23.0.0 | 172.23.0.63 |
| 172.23.0.0 | 172.23.0.63 |
| 172.23.0.0 | 172.23.0.63 |
Example 1: IP Addressing Configuration, R1
Interface F0/0 Ip address 172.23.1.1 255.255.255.192 ! interface serial 0/0/0 encapsulation frame-relay ! interface S0/0/0.1 point-to-point ip address 172.23.0.1 255.255.255.192 ! interface s0/0/0.2 point-to-point ip address 172.23.0.65 255.255.255.192 ! interface s0/0/0.3 point-to-point ip address 172.23.0.129 255.255.255.192
Example 2: IP Addressing Configuration, R2
Interface F0/0 Ip address 172.23.2.1 255.255.255.192 ! interface serial 0/0/0 encapsulation frame-relay ! interface S0/0/0.1 point-to-point ip address 172.23.0.62 255.255.255.192
Example 3: IP Addressing Configuration, R3
Interface F0/0 Ip address 172.23.3.1 255.255.255.192 ! interface serial 0/0/0 encapsulation frame-relay ! interface S0/0/0.1 point-to-point ip address 172.23.0.126 255.255.255.192
Example 4: IP Addressing Configuration, R4
Interface F0/0 Ip address 172.23.4.1 255.255.255.192 ! interface serial 0/0/0 encapsulation frame-relay ! interface S0/0/0.1 point-to-point ip address 172.23.0.190 255.255.255.192
