Subnetting Address Space on Cisco Packet Tracer Practice Hands On

[jws47]

Hello, I'm having trouble with this part of my Cisco Packet Tracer Practice Hands-On assignment:

Task 1: Subnet the Address Space.

Step 1. Examine the network requirements.

You have been given the 192.168.110.0/24 address space to use in your network design. The network consists of the following segments:

• The Fa0/0 connected to router R1 will require enough IP addresses to support 72 hosts. 
• The Fa0/1 connected to router R1 will require enough IP addresses to support 81 hosts.    

Here are some screenshots of what I have so far:

 

 

Everything pings properly, but I'm not totally sure I did it correctly in accordance with the directions. Does my network have enough IP addresses to support the number of hosts needed? Would anyone be able to confirm this for me and help out if necessary? Thank you.

Here's the rest of the directions for reference:

Step 2. Assign network addresses to the Topology Diagram.

Assign the First valid host address of each subnet to the router interfaces.

Assign the next valid host address of each subnet to PC’s.

Task 2: Configure the FastEthernet Interface with the appropriate IP Addresses.

Step 1: Configure the router interfaces.

Note: Remember the Port Status of the interface must be turned on (no shut).

Step 2: Configure the PC interfaces.

Configure the Ethernet interfaces of PC0 – PC1 with the IP addresses and default gateways from your network design.

Task 3: Verify the Configurations.

Answer the following questions to verify that the network is operating as expected.

From the PC1 attached to R1, is it possible to ping the default gateway?

From the PC0 attached to R1, is it possible to ping the default gateway?

Can you ping between all PC’s?

 

 

[Georg Pauwen]

Hello,

The Fa0/0 connected to router R1 will require enough IP addresses to support 72 hosts.

--> 192.168.110.0/25

--> Host Range 192.168.110.1 - 192.168.110.127

The Fa0/1 connected to router R1 will require enough IP addresses to support 81 hosts.

--> 192.168.110.128/25

--> Host Range 192.168.110.129 - 192.168.110.254

[Joseph W. Doherty]

When working with network sizes, keep in mind they change by powers of two, and how large each is.

In your case, you're given a /24 which has 256 IPs, one reserved for the network identification and the other reserved for that network's broadcast.  I.e. a /24 would have 254 available IPs for hosts.

A /24 is large enough for both your requirements of 72 and 81 hosts, but you also noted we want to subnet your /24.

A /24 contains two /25s, each of 128 IPs, again two IPs set aside.  Also again, each /25 can contain your 72 or 81 hosts requirement.

A /25 contains two /26s, each of 64 IPs, also again, two IPs set aside.  A /26 would NOT be large enough for either your host requirements. So, that's how you get to @Georg Pauwen's correct answer.

BTW a /24 has two /25s, or four /26s, or one /25 and two /26s, etc.

[jws47]

Thank you both for the responses. So here's the thing, when I enter in 192.168.110.129 for fa 0/1, I receive this error: 

For some reason when I enter in something like 192.168.111.1 for fa 0/1 everything runs smoothly. Do you guys know what I'm doing wrong in this instance?

[Joseph W. Doherty]

"Do you guys know what I'm doing wrong in this instance?"

Believe I do.

You need to use a /25 mask, you're still using a /24 mask.

I.e:

192.168.110.001 255.255.255.0 => 192.168.110.001 255.255.255.128

192.168.110.129 255.255.255.0 => 192.168.110.129 255.255.255.128

[jws47]

Alright, I changed both of my fa0/0 and fa0/1 subnet masks to 255.255.255.128 and it seems to be working perfectly. So basically using the /25 mask allows me to divide up the /24 mask I've been given to work with to allow for the host requirements (and utilizing a /25 would not violate the terms of the directions as written)? 

[Georg Pauwen]

Hello,

I think subdividing the /24 into two /25 subnets is exactly what they want you to do...

[jws47]

Perfect, that's everything I needed to know. Thank you both very much for your help!

[Joseph W. Doherty]

Yup.

BTW, problems of this type often require you to subnet an address block into many different block allocations to allow for different number of host requirements.

When doing this real world, you don't want to over allocate, as you can run out of address space because much of it is set aside but never used (hard to do if 10.0.0.0/8, but last company I worked at did run out [fortunately then came IPv6]), yet conversely, you often don't want to be too tight such than you need to later expand the address block.  Of course, you might be in a situation were number of hosts is known, and "cannot" be increased.  Like when you have a fixed number of desk spaces, so workstation hosts cannot increase.  Ha, but then something like VoIP phones come along and they need IPs too (often on their own network, but sometimes not, regardless, you'll need IPs for them).