Server with 2 NIC's on different Networks

I've had this issue before and couldn't really resolve it but it's come back to bite me, so...


I have an old Debian Server (also same issue on Slackware as it happens) which has two network cards, configured as below;
Code:
# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth6 eth7
iface eth7 inet static
address 23.128.28.20
netmask 255.255.240.0
network 23.128.16.0
# gateway 23.128.16.1

iface eth6 inet static
address 10.28.10.21
netmask 255.255.255.0
network 10.28.10.0
# gateway 10.28.10.1this result in a routing (route -n) table similar to;


Code:Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.28.10.0 0.0.0.0 255.255.255.0 U 0 0 0 eth6
23.128.16.0 0.0.0.0 255.255.240.0 U 0 0 0 eth7The idea is that this server can reach either of the 23. or 10. networks by means of eth6 and eth7 respectively and in the above state, traceroutes to IP addresses in those ranges seem to indicate that traffic is indeed going through the correct interfaces. However, I also need to receive traffic from both of those networks, each coming specifically through those interfaces and this I'm not so sure is reliable.
Additionally, there is a specific IP of 10.29.42.103 which resides on the network to which eth6 connects and I am specifically having trouble sending and more so, receiving connectivity from it. I've tried adding a static route with;


Code:route add 10.29.42.103 dev eth6 which then changes the routing table to something like;
Code:
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.29.42.103 0.0.0.0 255.255.255.255 UH 0 0 0 eth6
10.28.10.0 0.0.0.0 255.255.255.0 U 0 0 0 eth6
23.128.16.0 0.0.0.0 255.255.240.0 U 0 0 0 eth7 This does seem to help but I cant be 100%


Additionally, I have tried adding in the gateways for each of the interfaces (currently commented out above) and this changes the routing table to something like;


Code:Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.28.10.0 0.0.0.0 255.255.255.0 U 0 0 0 eth6
23.128.16.0 0.0.0.0 255.255.240.0 U 0 0 0 eth7
0.0.0.0 23.128.16.1 0.0.0.0 UG 0 0 0 eth7
0.0.0.0 10.28.10.1 0.0.0.0 UG 0 0 0 eth6This also has limited success but I still have possible issues and questions generally related to networking in this context. I feel as though I'm getting confused around the mask terms, as used when setting up an interface and when used in routing;

1. How exactly should I be setting up my server in order to be able to send and receive traffic from 2 networks as defined above?
2. I think understand that with networking, if an IP address falls within the network mask then it will not need the gateway to route the traffic, such that with 10.28.10.0 and 255.255.255.0, anything within that mask, will be sent directly, anything outwith that mask, will be sent to the gateway 10.28.16.1 to be routed, is this correct?
3. Following on from this then in the routing table, the Genmask, is that kind of the opposite from my understanding in point 2 above? Such that in order for traffic to to be routed through a specific interface, then the IP must fall within the Genmask?

Any help would be greatly appreciated.