Quiz

The access_log of apache web server ws2 shows:

128.198.1.48 - - [18/Sep/2002:18:00:27 -0600] "GET /index.html HTTP/1.1" 200 94
128.198.1.48 - - [18/Sep/2002:18:00:50 -0600] "GET /index.html HTTP/1.1" 304 -
128.198.1.48 - - [18/Sep/2002:18:00:56 -0600] "GET /index.html HTTP/1.1" 304 -

128.198.162.61 - - [18/Sep/2002:18:27:54 -0600] "GET /sales/sales.html HTTP/1.0" 200 107
128.198.162.61 - - [18/Sep/2002:18:27:56 -0600] "GET /sales/sales.html HTTP/1.0" 304 -

Why the some requests from the client are replied with code 304?