The ESP8266 is an amazing low cost device with embedded WiFi. I was unable to find a firewall for it so this is what I came up with. A simple firewall/access list.

In your sketch set the variable allowNet to the octets of the network you'd like to allow. Use one of the following formats. Only one network is supported as of now.

String allowNet = "192.168.1.149";
String allowNet = "192.168.1";
String allowNet = "172.16";
String allowNet = "10";

Then initialize the firewall object.

Firewall firewall;

In your main loop check to see if you passed the firewall.

int firewallResponse = firewall.checkFirewall(client.remoteIP().toString(), allowNet);
if ( firewallResponse == 0) {

There is a sample file included called ESP-Firewall.ino that will demonstrate the use.