Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I'm checking the stats of a busy site that host mirror downloads. I installed vnstat on the machine and after a few days i saw the number 125.25 Mbit/s or using -ru 0 15.29 MiB/s

I'm extremely confused. How is this possible? I assume he is on a 100mbps connection. He doesnt know his hosting stats. When looking at numbers i suspect MiB (the # seen using -ru 0) is 1000*1000 as oppose to 1024*1024. Ok but even if i do ((value*1000*1000)/1024)/1024 then *8 to get bits rather then bytes the result is still > 100Mbps (it was 116). Confused i googled. I found this wiki page and saw it meant 100mbps down and 50 up. Fantastic it makes sense.

Then i double check by doing -h and using the tx number * 1000 to get the # of bytes (it said KiB rather then Kib so i assume the number is bytes). Then i did /1024(for KB)/1024(MB) and then *8 for bits. The number was >100 (it was close to 150). The rx speed was insignificant (<1mbp).

How the heck is the network going so fast? Does this mean he has a connection >100mbps? I havent heard of that and heard of ISP buying speeds in 100mbps chucks. Is there some reason the number is so high? The server is running apache with php support. Its nothing special.

share|improve this question

I think what's going on is that vnstat reports the traffic at the Ethernet level. Every Ethernet packet contains some overhead to indicate where, on the Ethernet link, the packet must go. This overhead is 42 bytes long. The Ethernet payload typically consists of 16 bytes of IP header, 20 bytes of TCP header, and finally some data (the payload).

If your average payload size is N (in bytes), then the overhead of TCP over IP over Ethernet is N+78. I suspect that the 100Mb/s figure from the ISP counts the IP packet size, which is N+36. Seeing 125Mb/s of Ethernet for 100Mb/s of IP makes sense if N≈132.

share|improve this answer
interesting. Just to clarify something i am not talking about an ISP. I am talking about a server with a 100mbps port. If the port adds in the data and accepts 100mbps it would make sense but then how does vnstat see it? oh well it doesnt matter (if its true). That answer is pretty interesting. I dont know if the speed is maxed since when i visit the site to browse or to download for testing my speed is always fast and maxed. I have small pipes but low lag so i should notice if theres slowdowns but i havent yet – acidzombie24 Nov 20 '11 at 2:07
@acidzombie24 Hmm, I may have misunderstood your question. I find the network topology unclear. Who's downloading from whom and over what kind of network links? Where is that “100mbps” figure coming from (you meant megabytes per second, i.e. MB/s or MiB/s, right?) – Gilles Nov 20 '11 at 2:22
i'm not sure. Its a public site so anyone can download. I am looking at various numbers (such as average with and without -ru) and the value in -h. A google search of "dedicated server port speed" shows various mentions of 100mbps and mbps nor 100 is in that query. It seems standard. I remember reading about ISP buying speed in 100mbps chunks so i mentioned it. But i am looking at a dedicated server with public users downloading. – acidzombie24 Nov 20 '11 at 2:31

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.