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MiB = MebiByte = 1024*1024

MB = MegaByte = 1000*1000

ethtool returns speed = 1000MB/s ifconfig documentation indicated it returns units of MiB

How can I tell from that what is the percentage of capacity used? What is the realy max capacity of the network device? 1000 or 1024 MB?

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Ethernet speeds are in base 10 units (10000000, 100000000, 100000000, etc) so that correct unit would be based on SI prefixes. For some time the IT industry has been moving to using SI prefixes. The notable (and logical) exception is computer memory / cache which is based in binary.

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ifconfig talks only of its own output, specifically the lines like:

RX bytes:535806445879 (499.0 GiB) TX bytes:271709639024 (253.0 GiB)

Network device speeds are typically their physical bitrate in SI / 1000 units, and I can't recall any network tool doing otherwise.

Note the bit in bitrate. Depending on who you ask, that means using a small b, or spelling out 'bit' for clarity, or sometimes. Ethtool says 1000Mb/s, not 1000MB.

While only mentioned in passing, see sources like:

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so if I want to know the percentage of the adpater's utilization, should I take ifconfig response, multiply is by 1000^2 or 1024^2? And divide it by ethtool response, multiply by 1000^2 or 1024^2? – AYBABTU Jul 7 '13 at 7:07
Well, when it says something like "535806445879 (499.0 GiB)", note that 535806445879/(1024*1024*1024) is 499.0. So yes, you could just pick out the byte figure, subtract it from a figure you fetched a second or two ago, and divide that by the NIC speed. (I myself have a tool somewhere that does exactly that, to get an impression of speed on hosts where I can't install anything) – scarfboy Aug 9 '13 at 12:16

I believe the speed of devices and data transferred/stored are described differently. Ethernet (i.e., 10/100/1000/10000 Mbps) describes the speed of devices which is megabyte [MB] = 1,000,000 (10^6), and exactly the amount of data transferred or stored is mebibyte [MiB] = 1,048,576 (2^20)...

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