job screening question

Jon Lewis jlewis at
Fri Jul 6 00:14:59 UTC 2012

He'll have to come up with another weedout question, like "what's a /27?" 
I'm constantly amazed/disappointed when we interview candidates for a 
senior Linux admin job and they just don't know modern networking at all.

Even better question, with multiple right answers, "how many IPs are in a 
/32?"  You could probably have some fun with most applicants[1] when they 
answer 1, and then you ask "would you like to expand on that answer?"

The small (sub /24) subnets are dealt with so frequently in an ISP/hosting 
provider environment, that IMO, anyone claiming to have experience in such 
an environment should just flat out know how many IPs and the subnet masks 
for /32 - /24 in IPv4, or be sufficiently comfortable with subnetting that 
they can figure these things out quickly enough to avoid awkward pauses 
during the interview if asked about them.

1) At least the few who get it right.

On Thu, 5 Jul 2012, Mike Hale wrote:

> Something tells me you're suddenly going to find yourself with an
> influx of correct answers...
> On Thu, Jul 5, 2012 at 3:18 PM, William Herrin <bill at> wrote:
>> On Thu, Jul 5, 2012 at 5:05 PM, Derek Andrew <Derek.Andrew at> wrote:
>>>>> You implement a firewall on which you block all ICMP packets. What
>>>>> part of the TCP protocol (not IP in general, TCP specifically)
>>>>> malfunctions as a result?
>>> Isn't MTU discovery on IP and not TCP?
>> If you want to overthink the question, the failure in the TCP protocol
>> is that it doesn't adjust the MSS to match the path MTU. It continues
>> to rely on the incorrect path MTU estimate, sending too-large packets
>> which will never arrive. This happens because TCP doesn't receive a
>> notification that the path MTU estimate has changed from the default
>> because the lower layer PMTUD algorithm never receives the expected
>> ICMP packet.
>> This is, incidentally, is a detail I'd love for one of the candidates
>> to offer in response to that question. Bonus points if you discuss MSS
>> clamping and RFC 4821.
>> The less precise answer, path MTU discovery breaks, is just fine.
>> Regards,
>> Bill Herrin
>> --
>> William D. Herrin ................ herrin at  bill at
>> 3005 Crane Dr. ...................... Web: <>
>> Falls Church, VA 22042-3004
> -- 
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  Jon Lewis, MCP :)           |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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