Network Engineer Level 1 interview questions

massive_array_attack

Hey all!

I recently accepted an job interview, just to test the waters before CCNA. I think I made the interviewer angry

I did very well when the interviewer from the network side, asked me about ACLs etc. I told him that ACLs are being fired from top to bottom and there are always an invisible implicate deny at the bottom of the list, and I created an ACL off the top of my head.

Then they switch to a more server-side guy (Microsoft) to interview me.

He asked my what are the Class A, B, C sub net addresses are.

I told him Class A is 255.0.0.0, Class B is 255.255.0.0 and Class C is 255.255.255.0

He said I was wrong and it should start with 10.0.0.0 to 10.255.255.255.

I corrected him, by saying "Ohhh you mean by private IP address space RFC 1918". Then he got mad

TWX

One could make the argument that you weren't specific enough and he was flat-out wrong.

There are actual public IP ranges defined as Class-A ranges, of which 10.0.0.0/8 as private space sits among. The first bit of the IP address must be a zero (a zero in the binary digit that equals 128 in decimal) so the range of numbers with a zero in the first, most significant must range from 0 (which is reserved) through 127 (which is also reserved and used for localhost), or 00000000 through 01111111.

Class-B ranges of which 172.16.0.0/12 sits among (think of it as sixteen Class-B networks, 172.16.0.0/16 through 172.31.0.0/16) have a one in the first bit of the IP address and a zero in the second bit. This means that Class-B addresses can only be as low as 128 (1000000) or as high as the end of 191 (10111111).

Class-C ranges require the first two bits of the first octet to both be ones, and the third digit to be zero. This establishes a range of 192 (11000000) through the end of 223 (11011111).

Now, if I understand correctly, back when networks were entirely classful, what we now use as a subnet mask was not necessary as a given class of network could not be subdivided, and the arrangement of bits at the leading edge of the address dictated which octet was network and which octet was host. So, leading with a zero, that leading zero indicated that only the first octet was network, the other three octets were host. Leading with a one and zero, this indicated for Class-B networks that the first two octets were network, and the third and fourth octets were host. Leading with a one, a one, and a zero, this defined the Class-C network as having network for the first, second, and third octets, and that only the fourth octet was host.

This make me thing that subnet, as a term for subnet mask was specifically coined to explain the idea of the class no longer matching the network; that host bits or octets were now being used for below-class division, and that the mask itself was designed as a workaround to allow network classes other than the hard-fast rules of Class-A, Class-B, and Class-C networks.

I admit that I am not an expert on the early Internet, so if someone has better knowledge I bow to them.

james43026

Sounds like there was just miss-communication between you and the interviewer. To me, it sounds like he indeed wanted to know what IP addresses fall in the Class A, Class B, and Class C ranges. I don't thing he was asking for the subnet masks alone, as the question was for subnet addresses, which would imply that you need to provide an address range and the associated subnet mask for each class, but his answer of "He said I was wrong and it should start with 10.0.0.0 to 10.255.255.255" makes it sound like he only wanted private IP addresses listed. It all sounds a bit confusing, and strange. I would chalk it up to miss-communication. And in technical interviews, before you answer a question you aren't sure of, always ask clarifying questions, as they can help prove that you understand a subject just as much as the answer itself.

massive_array_attack

Thanks for the heads up guys. Hope I get more lucky when I complete my CCNA R&S

Justin-

Congrats on the new job bro! And good luck with your CCNA R&S !!!

Hondabuff

massive_array_attack wrote: »

Hey all!

I recently accepted an job interview, just to test the waters before CCNA. I think I made the interviewer angry

I did very well when the interviewer from the network side, asked me about ACLs etc. I told him that ACLs are being fired from top to bottom and there are always an invisible implicate deny at the bottom of the list, and I created an ACL off the top of my head.

Then they switch to a more server-side guy (Microsoft) to interview me.

He asked my what are the Class A, B, C sub net addresses are.

I told him Class A is 255.0.0.0, Class B is 255.255.0.0 and Class C is 255.255.255.0

He said I was wrong and it should start with 10.0.0.0 to 10.255.255.255.

I corrected him, by saying "Ohhh you mean by private IP address space RFC 1918". Then he got mad

You handled the situation wrong. You should of said, "I'm sorry, I must of misunderstood your question." We do this during our interviews to see how you handle the situation. Its part of the SAR's format. I will ask a question knowing that no matter how you answer it that I'm going to flip the answer. Trying to trump him with the RFC 1918 shows that you are not a team player. All he is thinking about is if he puts you on the production network and asks you to make a change will you a) Make the change and do as he says or b) Your going to be a "cowboy" and add your own touch to an acl or route statement. Use this as a life lesson for future interviews.

shortstop20

Hondabuff wrote: »

You handled the situation wrong. You should of said, "I'm sorry, I must of misunderstood your question." We do this during our interviews to see how you handle the situation. Its part of the SAR's format. I will ask a question knowing that no matter how you answer it that I'm going to flip the answer. Trying to trump him with the RFC 1918 shows that you are not a team player. All he is thinking about is if he puts you on the production network and asks you to make a change will you a) Make the change and do as he says or b) Your going to be a "cowboy" and add your own touch to an acl or route statement. Use this as a life lesson for future interviews.

This.

Show me a guy who likes to tell their interviewer that they're wrong and I'll show you a guy without a job.

JamesKurtovich

"Oh, you mean..." doesn't sound aggressive or arrogant to me. Maybe he has a temper? I wouldn't sweat one guy getting flustered if multiple people are questioning you though.

blatini

I think in the future if you want to "test the waters" you should probably just visit the IT Jobs forum. Lots of people have threads asking what questions to expect. I guess if you just want to get your feet wet with the in person interview what you did isn't too bad. But there are a lot better avenues of exploring what questions to expect in an interview. Depending where you are the IT field can be small, and networking people is everything. You don't want to leave the impression with someone you might run into in the future that you're a living potato