Which IP Address and Mask Work on a /30 Subnet?

200-301 By islever Editorial Last updated

Configure and verify IPv4 addressing and subnetting

Short answer

D 209.165.201.2 with the /30 mask 255.255.255.252 is the usable host address for the link.

Question

An engineer must configure a /30 subnet between two routes. Which usable IP address and subnet mask combination meets this criteria?
  1. interface e0/0 description to XX-AXXX:XXXXX ip address 10.2.1.3 255.255.255.252
  2. interface e0/0 description to XX-AXXX:XXXXX ip address 192.168.1.1 255.255.255.248
  3. interface e0/0 description to XX-AXXX:XXXXX ip address 172.16.1.4 255.255.255.248
  4. interface e0/0 description to XX-AXXX:XXXXX ip address 209.165.201.2 225.255.255.252 Correct Answer

Key Takeaways

You must identify which option both carries the /30 mask (255.255.255.252) and lists an address that is neither the network nor the broadcast identifier — the trap is picking a broadcast address or a non-/30 mask.

This question tests IPv4 subnetting for a /30 point-to-point link, which uses mask 255.255.255.252 and yields exactly two usable host addresses. Option D is correct because 209.165.201.2 falls inside the usable range of a properly formed /30 block, while the other choices either use a /29 mask or a broadcast address.

Common mistake: Most learners choose option A because its mask 255.255.255.252 is a genuine /30, but 10.2.1.3 is the broadcast address of 10.2.1.0/30 and cannot be assigned to an interface; others pick B or C without noticing the mask is 255.255.255.248 (/29), not /30.

Expert Analysis

Why the Answer Is Correct

Option D pairs the address 209.165.201.2 with what is clearly intended to be the /30 mask 255.255.255.252 (the printed value 225.255.255.252 is a typo acknowledged across the community). With that mask the block size is 4, so the network is 209.165.201.0, the broadcast is 209.165.201.3, and the usable host addresses are 209.165.201.1 and 209.165.201.2 — exactly the two addresses a /30 link needs. Because 209.165.201.2 is a valid assignable host address in that range, it satisfies the requirement of a usable IP on a /30 subnet.

Why the Other Options Are Wrong

Option A does use a true /30 mask of 255.255.255.252, but 10.2.1.3 is the broadcast address of the 10.2.1.0/30 network, so it can never be assigned to an interface — as Samitha and DatBroNZ both demonstrated by computing the block. Options B and C carry mask 255.255.255.248, which is a /29 (eight host addresses), so neither qualifies as a /30 subnet regardless of the address used. Option D's only flaw is the transposed first octet 225 instead of 255, a typo the community unanimously corrected while still selecting it as the intended answer.

Community Comment Notes

Several commenters converged on D while flagging the mask typo: Ali526 wrote "There is typo in D; should be 255.255.255.252" and ZUMY added "Answer is D. But there is a typo in mask and should be 255.255.255.252". Aleksos pushed back that "D is wrong due to bad mask - 225.255.255.252 - first octet is 2 2 5!" while confirming A is the broadcast address. DatBroNZ walked through all four options and concluded A is the broadcast, B and C are /29, leaving D. Mr_reyes raised a valid side note that Cisco routers also support /31 on point-to-point links, but that does not change which listed option best fits the /30 requirement.

Exam Strategy

On subnetting questions, first eliminate every option whose mask does not match the requested prefix, then check that the remaining address is neither the network ID nor the broadcast ID. Expect occasional typos in exam dumps and choose the option that is correct modulo the obvious error.

Frequently Asked Questions

Why can't I use 10.2.1.3 on a /30 subnet in option A?

In 10.2.1.0/30 the addresses are network .0, hosts .1 and .2, and broadcast .3, so .3 cannot be assigned to an interface.

Are options B and C /30 subnets?

No, both use 255.255.255.248, which is a /29 mask giving eight addresses, not the /30 required.

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Discussion 77 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 30 of 77

  1. rugginic 👍 64
    answer is D. The up in A is a broadcast
  2. r8derfan33 👍 32
    Wait? What? Did you look at the subnet mask? 225.255.255.252?
  3. Samitha 👍 16
    First poit is /30 subnet is 255.255.255.252.so we can filter out A and C. Let find the block size---> 256-252=4 From 4 IPs one for network and one for broadcast.Two usable IPs remain. So 209.165.201.0 is the network and 209.165.201.2,209.165.201.3 are the usable IPs. So 209.165.201.2 is a useble IP with 255.255.255.252 subnet mask. Answer is D.
  4. Ali526 👍 8
    There is typo in D; should be 255.255.255.252. Having said that, it's the correct answer.
  5. DatBroNZ 👍 7
    Option D is the correct (the question has a typo, mask is 255.255.255.252) Network: 209.165.201.0 Broadcast: 209.165.201.3 Usable IPs: 209.165.201.1 - 209.165.201.2 Option A not correct because that IP is broadcast Network: 10.2.1.0 Broadcast: 10.2.1.3 Usable IPs: 10.2.1.1 - 10.2.1.2 Option B and C are wrong because they have a /29 mask
Show 25 more comments
  1. Brice 👍 6
    It is à mistake. Subnet mask /30 is 255.255.255.252
  2. mr_reyes 👍 5
    You guys know that Cisco supports /31 CIDR for point-to-point links, right? Yes, in theory, the study guides tell us that /30 is the highest possible CIDR with only two usable addresses. But in reality, /31 can be used and there is no broadcast there. Look it up on the Cisco forums. With that said, the answer would depend on whether there is a typo in the question or not! For this particular question is A ( which is a broadcast indeed ), simply because 225.255.255.252. in no a VALID subnet mask! But, if answer D has a typo and the subnet mask is actually 255.255.255.252, then the correct answer is definitely D! Be careful when you take the actual exam.
  3. ZUMY 👍 5
    Answer is D. But there is a typo in mask and should be 255.255.255.252 In a /30 Subnet There Count will be Host (2^2-2)=4-2 1st address is network and the last is broadcast As given 209.165.201.2 N/W ID 209.165.201.0 Fist IP 209.165.201.1 Second/Last IP 209.165.201.2 Broad Cast ID 209.168.201.3
  4. dave1992 👍 5
    ill simplify the answer. its D because a /30 means 2 host bits left over =2^2=4. subtract 2 to give us 2 usable host ip addresses. and based on the mask of a /30 = .252 the only thing that meets the fields is D
  5. Marcelious 👍 5
    I thought a subnet of 30 would have all 1's on the first part of the binary for 30 bits? So how can it start 225? This doesn't make sense to me with my recent knowledge.
  6. aleksos 👍 5
    D is wrong due to bad mask - 225.255.255.252 - first octet is 2 2 5! And A is also wrong - broadcast IP
  7. Tag 👍 5
    /30 subnet is 255.255.255.252
  8. doctot_tlc 👍 4
    The Answers B and C are out because their submask have the prefix /29, in A the 10.2.1.3 is the broadcast of 10.2.1.0 network. The answer correct is D because 209.165.201.2 is the 2nd available IP for 209.165.201.0 network.
  9. therandman 👍 4
    This question isn't hard. First of all, the /30 mask must be identified. Ie, 255.255.255.252. This leaves options. The address should also be a host address and only D satisfies that requirement.
  10. dendio 👍 4
    Right, A is the broadcast address - it is explained in better detail here:
  11. 02092020 👍 3
    If this exact question is on the actual exam I choose answer A. It is not the correct answer but it is the best answer available. /30 leaves us at 255.255.255.252 so A is the only (!!) answer. Technically, answer D is a 26 bit subnet (in a weird sequence). The questions is not "An engineer must configure a /30 subnet between two routes on a router.." If it was, then answer A would be counter intuitive because answer A is not a valid entry on a router. 10.2.1.3 in a /30 subnet is indeed a valid IP address and therefore usable as the braodcast address. It is not a valid host address. If the exam had answer D as 255.255.255.252 instead of 225.255.255.252 then I would choose D. We all can safely assume that D has a typo in the subnetmask.
  12. sunytech 👍 3
    Answer is D Here is how I do my host and subnets /30 borrows six(6) bits 24+6=30 bits 1 2 3 4 5 6 7 8 subnets: 128 192 224 240 248 252 254 255 hosts: 128 64 32 16 8 4 2 1 *The way I get the subnets is by adding the second host and first subnet - 128+64=192 192+32=224 etc etc since this is a /30 the 6 bits falls under /252 and 252 falls with 4 hosts but, you can't use all 4 hosts. You need one for Network and One for broadcast and you are left with 2 host. Network Valid Host Broadcast 0 1-2 3 4 5-6 7 8 9-10 11 12 13-14 15 16 etc etc If you learn the above formula you will be able to do all the subnets/host in your head. :-)
  13. Techslash40 👍 3
    The intended answer could only be D except the mac address is erroneous. You can't intermix ones and zeroes In the subnet masks. If there are to be 1s, they will all be left and not mixed. 225 is mixed, thus the subnet mask for d is wrong. A is wrong because it's the broadcast address.
  14. uevenasdf 👍 3
    - 225.255.255.252 is a /30 subnet so that rules out B/C - 10.2.1.3 is a broadcast address for 10.2.1.0-3/30 subnet so A is wrong (.3 = 000000{11} /30) Which leaves D 209.165.201.2 which is a host address for 209.165.201.0-3/30 subnet (.2 = 000000{10} /30)
  15. SiliconeIT 👍 3
    The answer is A, I saw some saying that A is a broadcast address and would like to remind everyone that broadcast addresses can be reconfigured while the mistake of having subnet mask 225.255.255.251 eliminates it as a candidate
  16. therandman 👍 3
    This question isn't hard. First of all, the /30 mask must be identified. Ie, 255.255.255.252. This leaves 2 options. The address should also be a host address and only D satisfies that requirement.
  17. Nhan 👍 3
    Between 2 router, is point to pint connection, which mean you can use /30 or even /31, google YouTube video of Jeremy you will see the the explanation in deep.
  18. arpee 👍 3
    Yes A is a broadcast address because relevant bits (the last two, because it is a /30 subnet) are both 1 - that makes it a broadcast address.
  19. marcojmnez 👍 3
    10.2.1.3 255.255.255.252 is broadcast, still there's a typo in D but the right one anyways.
  20. yewastedmytime 👍 3
    Just update the small typo on the answer? Will save the un-needed drama
  21. Oviedo 👍 3 Answered A
    Subnet mask for D is incorrect and A is using the broadcast IP
  22. il_pelato_di_casalbruciato 👍 3
    please modify the subnet in D answer, the correct one.
  23. Ettmoh 👍 3
    correct answer is D. @ A is a broadcast @
  24. Dolph 👍 3
    This is just typo. Correct answer is D!
  25. Tengereni 👍 3
    thats a typo

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