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skip, seek and count default to counting the number of blocks. So if you want Megabytes 50-60 of a file you can do either a 1 byte block size:

> dd if=/tmp/file1 bs=1 skip=50000000 count=10000000

or you can use any block size you want and treat skip and count as byte counts, not block counts.

> dd if=/tmp/file1 bs=4M skip=50M count=10M iflag=skip_bytes,count_bytes

Option 1 will run at Kilobytes per second as it is transferring 1 byte at a time (my test gives me 933kB/s)

Option 2 will run a 100's MB per second as it is transferring 4MB at a time (my test gives me 202MB/s).

Edit: What I should have said at the top of the post was "If you want to extract data from a file whose size is not an integer block multiple". In the above example you could have used a Blocksize of 10MB for the same result. You cannot do this for an oddly size extraction (say 1234567bytes).

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dd now counts bytes instead of blocks if a block count ends in "B". For example, 'dd count=100KiB' now copies 100 KiB of data, not 102,400 blocks of data. The flags count_bytes, skip_bytes and seek_bytes are therefore obsolescent and are no longer documented, though they still work.