Solved: Finding last/first usable IP Addresses in subnet

mmmartin823 · ‎04-25-2016

Hello there,

I am currently studying for my CCENT exam happening next month and I am struggling to understand the process for finding the explanation for the answers to these two practice questions below. Can you please assist me with your process on how you can arrive at the correct answer as listed below? (...I've been just coming up with the wrong answer recently with either "254" in the last octet for broadcast or "1" for the subnet ID) Thank you in advance!

Question #1:
A fellow engineer tells you to configure the DHCP server to lease the last 100 usable IP addresses in subnet 10.1.4.0/23. Which of the following IP addresses could be leased as a result of your new configuration?

Answer: 10.1.5.200

Question #2:
A fellow engineer tells you to configure the DHCP server to lease the first 20 usable IP addresses in subnet 192.168.9.96/27. Which of the following IP addresses could be leased as a result of your new configuration?

Answer: 192.168.9.110

Pawan Raut · ‎04-25-2016

Hi

Its very simple and important topic in networking.

1) Subnet 10.1.4.0/23 has subnet mask as .255.255.254.0 so the IP address started from 10.1.4.0 and ending with 10.1.5.255 as you know first IP address is network address that is 10.1.4.0 and last address is broadcast IP address 10.1.5.255 and we cant assigned network and broadcast IP address so the usable IP address starting with 10.1.4.1 and ending with 10.1.5.254

so the last 100 IP address in the range 10.1.4.1 and ending with 10.1.5.254 is between 10.1.5.155 to 10.1.5.254.

2) Subnet 192.168.9.96/27 has subnet mask as 255.255.255.224 so the IP address started from 192.168.9.96 and ending with 192.168..9.127 as you know first IP address is network address that is 192.168.9.96 and last address is broadcast IP address 192.168.9.127 and we cant assigned network and broadcast IP address so the usable IP address starting with 192.168.9.97 and ending with 192.168.9.126

so the first 100 IP address in the range 192.168.9.97 and ending with 192.168.9.126 is between 192.168.9.97 to 192.168.9.116

View solution in original post

Pawan Raut · ‎04-25-2016

Hi

Its very simple and important topic in networking.

1) Subnet 10.1.4.0/23 has subnet mask as .255.255.254.0 so the IP address started from 10.1.4.0 and ending with 10.1.5.255 as you know first IP address is network address that is 10.1.4.0 and last address is broadcast IP address 10.1.5.255 and we cant assigned network and broadcast IP address so the usable IP address starting with 10.1.4.1 and ending with 10.1.5.254

so the last 100 IP address in the range 10.1.4.1 and ending with 10.1.5.254 is between 10.1.5.155 to 10.1.5.254.

2) Subnet 192.168.9.96/27 has subnet mask as 255.255.255.224 so the IP address started from 192.168.9.96 and ending with 192.168..9.127 as you know first IP address is network address that is 192.168.9.96 and last address is broadcast IP address 192.168.9.127 and we cant assigned network and broadcast IP address so the usable IP address starting with 192.168.9.97 and ending with 192.168.9.126

so the first 100 IP address in the range 192.168.9.97 and ending with 192.168.9.126 is between 192.168.9.97 to 192.168.9.116