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Subnetting Questions

November 6th, 2017 Go to comments

Note: If you are not sure about Subnetting, please read our Subnetting Tutorial – Subnetting Made Easy.

Question 1


Network A needs 120 hosts < 128 = 27 -> Need a subnet mask of 7 bit 0s -> “/25″.

Because the ip subnet-zero command is used, network can be used.

Answer E “Link A –″ is not correct because this subnet belongs to MARKETING subnet (
Answer F “Link A –″ is not correct because this subnet belongs to ADMIN subnet (

Question 2


Although all above answers are correct but is the best choice as it is the most specific prefix-match one.

Question 3

Question 4


We need to summarize 4 subnets so we have to move left 2 bits (22 = 4). In this question we can guess the initial subnet mask is /24 because,,, belong to different networks. So “/24″ moves left 2 bits -> /22.

Question 5


From the subnet mask of (/28) we learn there are 24 – 2 = 14 hosts per subnet.

Question 6


From the subnet mask of we learn that the increment is 8 therefore is a network address which cannot be assigned to a host. Other network addresses are,,… Notice that is a valid host address (which belongs to to subnet).

Question 7


“/25” means 1111 1111.1111 1111.1000 0000 in binary or in decimal.

Question 8

Question 9


The principle here is if the subnet mask makes two IP addresses and in the same subnet then the Network device A does not need to have IP addresses on its interfaces (and we don’t need a Layer 3 device here).

A quick way to find out the correct answers is notice that all 255.255.255.x subnet masks will separate these two IP addresses into two separate subnets so we need a Layer 3 device here and each interface must require an IP address on a unique IP subnet -> A, C are not correct while B, D are correct.

With subnet mask, the increment here is 2 in the third octet -> the first subnet is from to, in which two above IP addresses belong to -> each interface of Network device A does not require an IP address -> E is correct.

Question 10

Question 11


The RFC 1518 is Classless Interdomain Routing (CIDR). CIDR is a mechanism developed to help alleviate the problem of exhaustion of IP addresses and growth of routing tables.

The problems were:

+ With the classful routing system, individual networks were either limited to 254 hosts (/24) or 65,534 hosts (/16). For many network enterprises, 254 hosts were not enough and 65,534 were too large to be used efficiently.
+ Routing information overload. The size and rate of growth of the routing tables in Internet routers is beyond the ability of current software (and people) to effectively manage.
+ Eventual exhaustion of IP network numbers.

To solve these problem, CIDR was selected as the solution in 1992.

In contrast to classful routing, which categorizes addresses into one of three blocks, CIDR allows for blocks of IP addresses to be allocated to Internet service providers. The blocks are then split up and assigned to the provider’s customers.

According to the CIDR standard, the first part of an IP address is a prefix, which identifies the network. The prefix is followed by the host identifier so that information packets can be sent to particular computers within the network. A CIDR address includes the standard 32-bit IP address and also the network prefix. For example, a CIDR address of, the “/26” indicates the first 26 bits are used to identify the unique network, leaving the remaining bits to identify the specific hosts.

Therefore, instead of assigning the whole block of a class B or C address, now smaller blocks of a class can be assigned. For example, instead of assigning a whole block of, a smaller block, like or, can be assigned.

Comments (21) Comments
  1. jasin
    March 6th, 2017

    You have been asked to come up with a subnet mask that will allow all three web servers to be on the same network while providing the maximum number of subnets. Which network address and subnet mask meet this requirement?
    any one please could explain how we goina work out for 3 web server

  2. skyflakes
    March 7th, 2017

    We want to have 3 host right or 3 web servers? so B is correct because of the subnet. or /29. the increment of it is 8. 8 – 2 = 6 usable host, so it would fit the requirements,

  3. Aaron
    March 7th, 2017

    8 – 2 because we subtract the network id and broadcast id. So we could only use 6 host, unlike in letter C for instance is so it’s 30. the increment is 4 so 4 – 2 = 2 host only, so it wouldn’t match the needed requirements for 3. i hope my answer is correct. :)

  4. Eriol
    March 9th, 2017

    Both Skyflakes and Aaron are correct (: because you need to use a mask of /29 to have 6 usable IP’s to configure your “3” needed host, the masks /30 only gives you 2 usable IPs which doesn’t meet the requirements of 3 hosts and the /28 mask gives you 14 usable IPs, which doesn’t fullfil the “while providing the maximum number of subnets” statement.

  5. Hicham
    April 4th, 2017

    Sorry i don’t see quetions???

  6. Henchman
    April 7th, 2017

    Just at first glance of your answers I would say either ” B ” or ” C ” could be used, but then I had to look at your Subnet Mask in the last octet, and knowing that I need to provide for 3 Web Servers and give them IP Addresses from my subnet mask, I would have to borrow 5 bits from left to right giving me a mask ending with 248, i.e, and the 5 borrowed bits will give me 8 host minus the broadcast and IP so I will have 6 usable IP addresses for the 3 Web Servers leaving me with 2 left over IP for other devices……that is how I would do it !!!!!

  7. Vince
    April 30th, 2017

    Why do you say subnet hosts. This kind of vocabulary is common among people who don’t know how the technology is used correctly.

  8. Ricky
    May 13th, 2017

    Need help#
    Suppose You have a big company of 63 hosts per network. You are the System Administrator of your company. You have to plan the addressing scheme. Your are given IP And your Dept. (Engtneering) uses Subnet N 399.
    Q. What will be the Subnet bit?

  9. MUhammad Mohyuddin
    May 16th, 2017

    You have been asked to come up with a subnet mask that will allow all three web servers to be on the same network while providing the maximum number of subnets. Which network address and subnet mask meet this requirement?


  10. davegi
    May 22nd, 2017

    why the payment method is only paypal? any other options like mastercard and others

  11. Thaddeus
    June 28th, 2017

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  12. Anonymous
    June 30th, 2017

    the question doesn’t appear,what’s the problem?

  13. idiott
    July 16th, 2017


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  14. Anonymous
    August 21st, 2017

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    Thanks tut, on to ccnp now

  15. anon
    September 10th, 2017

    Question 10

    A & B are equally valid answers.

    Either change the subnet mask to or smaller
    or change the gateway address so the last octet is between 25-33.

    either will fix the problem.

  16. Question 10 -Anon
    October 24th, 2017

    The reason why the answer to Question 10 is A is because the subnet of the host is .240, on a network. The usable IP’s are 17-30 (31 is broadcast, and 32 is the next network). The gateway is on You need L3 to get to it. Answer B “The default gateway address for Computer A is incorrect” refers to the “gateway” address on the host…no the IP of the router.

  17. Adam
    November 8th, 2017

    Subnettning is very important… we need more examples;)
    you can find more examples also here

  18. nakajims
    December 14th, 2017

    Question 11 is asking RFC1918 which is IPv4 private address. But explanation mentiond RFC1518.??

  19. mohamoud sh
    January 8th, 2018

    please can you help me today i want to attend an exam about network adminstrator

  20. cthelite
    January 17th, 2018

    RFC 1518 is considered to be the first solution to IPv4 address exhaustion… it created CIDR… but didn’t solve any issues with the growing global routing tables, so answer “A” only would apply to RFC 1518…

    RFC 1519 specifically dealt with CIDR AND Address Assignment and Aggregation Strategies, so if that was the question, “A” and “D” would likely be the answer…

    But the question is about RFC 1918 and neither of these previous RFCs did anything to increase the number of publicly attachable hosts beyond what can be accommodated by a 32-bit addressing scheme, which was the ultimate limit to IPv4.

    So RFC 1631 (NAT) and subsequently 1918 (Private Addresses) did what the other two could not… now you could scale the number of nodes attached to the Internet to literally trillions of devices… still a workaround, a bandaid, but this arguably did the most to alleviate the impending IPv4 address exhaustion…

    Sooo, “A” and “B” dealing with IP addresses and their limitation (running out and overlapping) are the two most correct answers for the two reasons for RFC 1918.

    “C” is obviously not correct as it pertains to IPv6… and as discussed “D” would apply more to RFC 1519… “E” is not a good answer because private addresses don’t “support” NAT, but rather NAT supports the use of private addresses on the internet.

    So best answer is “A” and “B”.

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