Subnet Configuration Problème

[superkakaboum]

Hello,
I'm Gibril, a french student in networks and telecomunication, and we had to choose a subnet for our ethernet shield, but it didn't work (In the code (IPAddress subnet...) if I wanted it to be in /28 . I don't know why we need to change the W5100.cpp file, but also to change the subnet we need to go to the Ethernet .cpp file, line 75:

void EthernetClass::begin(uint8_t *mac, IPAddress ip, IPAddress dns, IPAddress gateway)
{
IPAddress subnet(255, 255, 255, 240); <--- Change this part
begin(mac, ip, dns, gateway, subnet);
}

I'm not very good at programming and maybe I don't know how to use the arduino board but I'd like it to work in my arduino code when I set the subnet and whatever I want it to work so I don't have to go into the configuration files.

For the record, I had to carry out a project to retrieve data from an angular sensor via a more or less complex network on gns3, a storage server which would then display the statistics on a web page.

[JAndrassy]

there is void begin(uint8_t *mac, IPAddress ip, IPAddress dns, IPAddress gateway, IPAddress subnet); too