Objectives

Part $1$: Subnetting the $4$th Octet

Part $2$: Configure Routers with IP addresses and static routes

Part $3$: Configure the PCs

Part $4$: Verify Connectivity

Background

In this lab, you will subnet the IP block listed in the instructions. All end user networks will be subnetted based on how many hosts are required for the network. Once you have calculated the correct subnets to meet the network requirements you will configure the routers and PC$\text{'}$s with the network information.

Part $1$: Subnetting the $4$th Octet

The available IP space to subnet is $172\mathrm{.}16\mathrm{.}0\mathrm{.}0255\mathrm{.}255\mathrm{.}255\mathrm{.}0.$ Subnet three contiguous networks based on the host requirements using the available IP space while using as few IPs as possible. Host requirements are located on the PT network diagram

Step $1$: Identify the largest required network

In this lab, the largest host network is $120$ hosts

Step $2$: Size the Subnet Mask to meet the requirement

We use the Binary Positional table below to size it correctly. $128$ is bigger than the host requirement of $120$ so it gets a $1$ under it$.$ Positions $64$ through $1$ are smaller than $120$ so they get $0\text{'}$s$.$ Yellow is the network portion of the IP address $($the subnet mask$).$ Green is the host portion of the IP address $($what you can assign to a computer$).$

Hint: Remember the smallest number with a $1$ under it is the Magic Number. In this network, the magic number is $128$ as it was the only number with a $1$ under it$.$ We'll use it in a few steps

$128$

$64$

$32$

$16$

$8$

$4$

$2$

$1$

$1$

$0$

$0$

$0$

$0$

$0$

$0$

$0$

Step $3$: Now add up all the numbers with a $1$ under it

Only $128$ has a $1$ under it so our subnet mask is $255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

Subnet Mask

Network ID

First Usable IP

Last Usable IP

Broadcast IP

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

Step $4$: Calculate the Network ID

The network ID is always the first IP address. The subnet space we have to work with is $172\mathrm{.}16\mathrm{.}0\mathrm{.}0255\mathrm{.}255\mathrm{.}255\mathrm{.}0.172\mathrm{.}16\mathrm{.}0\mathrm{.}0$ is the first IP address available so it is our network ID

Subnet Mask

Network ID

First Usable IP

Last Usable IP

Broadcast IP

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}0$

Step $5$: Calculate the First Usable IP$.$

The First Usable IP is always the IP address if you add $1$ to the network ID$.$ Our network ID for this network is $172\mathrm{.}16\mathrm{.}0\mathrm{.}0.$ Now add $1$ to the fourth octet and we get $172\mathrm{.}16\mathrm{.}0\mathrm{.}1.$ Remember the first three octets $172\mathrm{.}16\mathrm{.}0$ stay the same as we are subnetting the fourth octet

Subnet Mask

Network ID

First Usable IP

Last Usable IP

Broadcast IP

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}0$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}1$

Step $6$: Calculate the Broadcast IP

The Broadcast IP will always be the last IP address of a network. Recall the Magic Number from Step $2.$ A simple way to figure out the Broadcast IP is to take your Magic Number and add that number to your network ID$,$ in the same octet that the magic number is located in$.$ Then subtract $1$ from that number. In this network, add $128$ to your network ID $=172\mathrm{.}16\mathrm{.}0\mathrm{.}128.$ Now subtract $1$ from that number $(128$ minus $1=127).172\mathrm{.}16\mathrm{.}0\mathrm{.}127$ is the Broadcast IP

Subnet Mask

Network ID

First Usable IP

Last Usable IP

Broadcast IP

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}0$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}1$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}127$

Step $7$: Calculate the Last Usable IP

The Last Usable IP will always be one less than the broadcast IP$.$ Broadcast IP $=172\mathrm{.}16\mathrm{.}0\mathrm{.}127$ minus $1=172\mathrm{.}16\mathrm{.}0\mathrm{.}126$

Subnet Mask

Network ID

First Usable IP

Last Usable IP

Broadcast IP

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}0$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}1$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}126$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}127$

Step $8$: Repeat for the two other networks

Hint: The next network ID is $1$ more than the broadcast IP of this network

Use the table below to help calculate the Subnet Mask, network ID$,$ first & last usable, and the Broadcast IP

Subnet Mask

Network ID

First Usable IP

Last Usable IP

Broadcast IP

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}0$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}1$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}126$

$172\mathrm{.}16\mathrm{.}0\mathrm{.}127$