Added configurations for main-site linux host and off-site linux host

Added GNS3 example topology image

main-site/isc-dhcp/isc-dhcp-server

@@ -0,0 +1,20 @@
+# /etc/default/isc-dhcp-server
+
+# Defaults for isc-dhcp-server (sourced by /etc/init.d/isc-dhcp-server)
+
+# Path to dhcpd's config file (default: /etc/dhcp/dhcpd.conf).
+#DHCPDv4_CONF=/etc/dhcp/dhcpd.conf
+#DHCPDv6_CONF=/etc/dhcp/dhcpd6.conf
+
+# Path to dhcpd's PID file (default: /var/run/dhcpd.pid).
+#DHCPDv4_PID=/var/run/dhcpd.pid
+#DHCPDv6_PID=/var/run/dhcpd6.pid
+
+# Additional options to start dhcpd with.
+#	Don't use options -cf or -pf here; use DHCPD_CONF/ DHCPD_PID instead
+#OPTIONS=""
+
+# On what interfaces should the DHCP server (dhcpd) serve DHCP requests?
+#	Separate multiple interfaces with spaces, e.g. "eth0 eth1".
+INTERFACESv4="bridge0"
+INTERFACESv6=""