bandwidth issue

[santoshdj123]

hi all
i am using 256kbps line. i have one linux server which is running 24 hours a day. from 14-15 days when client machine were not using internet connection then the bandwidth use is shown as 46kbps download and o kbps upload. what may be the reason when no one client machine is using internet at that time.
i use tcpdump on eth0 it is showing some transactions. (showing me some public ips)

 

[John Lewis]

Anything coming inbound will incread your download, whether you ask for it or not.

Broadcasts and other internet noise that makes it's way to your box will increment the download usage. Your box generally will not respond, so you don't get a corresponding upload.