If it's a regular ethernet router then pools will definitely see each miner's own hashrate, because each miner should have its own worker name and the router knows nothing about ASICs.
In each miner's settings, when you connect to its management console via LAN, you can change the worker name, so if you wanted to show all the hashrate combined you'd simply edit the worker name to be the same for all miners (but I do not recommend this because it becomes harder to see when an ASIC dies).
so when a farm is connected to a pool , this one knows all hashrates of each miner ?
Depends.
If they use a proxy, as any farm would unless they've got no idea what they are doing, then only the proxy connects to the pool.
So that would be no.
To the pool, the total amount of data coming and going from a proxy is the equivalent to a single miner.
The proxy gets work from the pool, then each miner talking to the proxy get a slightly modified version of that work.
So as example, with 100 x 100TH/s miners, the pool will see the proxy as a single 10PH miner and thus set it's work difficulty so it sends the same number of shares as a 100TH/s miner, but the shares are worth about 100 times as much each.
This also, of course, makes the connection from the farm to the pool faster.
Instead of having to send 100 work items to the farm every work change, it sends only one work item.
Instead of the farm sending 100 shares, on average, over the time a single miner is expected to find a share, it sends, on average, one share.
If you are unsure about how shares and difficulty work, read here:
https://kano.is/index.php?k=workdiff
