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Subnetting Questions

November 6th, 2018 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 (6) Comments
  1. Vijay
    December 15th, 2019

    Can someone explain Q 4 to me?

    December 18th, 2019

    Thanks 9tut!!, I went through more than 900

    OSPF, ACL and DHCP labs
    study the new questions 1-6 this is 85 percent of the exam

  3. Adam999
    January 21st, 2020


    Where to find those 1-6 new questions ?

  4. @Adam999
    January 22nd, 2020

    On the right top of this site. Premium Membership is required and it worth!

  5. Anonymous
    February 17th, 2020

    @9tut love the content..really helping give confidence through the labs and seeing how questions are structured + the tutorials and explanations are a bonus! . Many thanks.

    If anyone is struggling with subnetting – try subnettingpractice dot com
    Absolutely worth a look as an additional resource for practising + has a few other areas all seem to be aimed at CCNA level topics.

    Good luck to everyone!

  6. Arun
    July 3rd, 2020

    Can Anyone explain question2 please. it is confusing.

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