Thanks Cyril, maybe i wasn't explaining well : all is fine connected and was working fine till i've replaced the internet router who is just connected on the port 1 of the first mac. The other macs are on etnet hub (fixed address) and all is fine.
The problem i've got is : if the mac 1 is connected to the internet router i just get a slow sharing, if i remove the etnet cable ( port 1 attached to the internet modem ) all is fast and fine.