MacTCP is a fully-compliant TCP/IP (and thus UDP/IP) driver. Your SMC Router (Model??) should have no trouble on this account, unless it was specifically built for DOSBOXES (in which case you're probably hosed).
The signal point to me seems to be a bit of confusion in your network topology. LocalTalk is an AppleTalk hardware-layer interface; it handles AppleTalk connections across LocalTalk physical media (a 230Kbs paired TX-RX serial line connection terminated at its distant ends). There are many LocalTalk implementation---PhoneNet was very popular and inexpensive, Apple made its own LocalTalk physical media, and so did some others.
EtherTalk is also an AppleTalk hardware-layer interface; it handles AppleTalk connections over Ethernet physical media (10Base-2, 10Base-5, 10Base-T, 100Base-TX, 10/100Base-TX, etc..). There is even a FDDITalk and of course, TokenTalk.
ALL of these are AppleTalk networks, even those that use Ethernet physical media.
From what you describe, it would appear your Quadra is connected to two physically discrete networks employing different software architectures; one AppleTalk network---eMate to Quadra---and one TCP/IP network---Quadra to Backbone.
Unless you have actual Macintosh routing or bridging software, or A/UX with Packet Forwarding enabled, your Quadra will NOT route data between the two Networks.
It would be helpful if you could tell me what information you discover from opening the MacTCP and Network Control Panels---the full IP Address, the full network mask (not '.xxx' please), the full router IP, DNS server info (unless that is publicly accessible, of course, in which case you have another problem), Domain Name info, and any Hosts file info (for MacTCP), and which Interface is selected (in the the Network CP).
DHCP is problematic for small networks, particularly if they connect to wider LANs or WANs (like your ISP). While useful for some applications, it is a lot of trouble for small networks. Unless you have a compelling reason to employ DHCP, don't; it makes Network troubleshooting very difficult unless you have control over every aspect of the network, have all the complimentary software, and lots of bandwidth to burn (which your Quadra certainly does NOT---it can maybe hit 1Mb/s throughput on Ethernet, which is still better than a DOSBOX).
I do not believe MacTCP is your problem. I run MacTCP in a multi-vendor IP environment with no problems---MacTen, OpenTransport, 10.4, AIX 4, A/UX---trouble-free. I suspect you have TWO problems interfering with one another---a confused Quadra, and an attempt to route/bridge from the Quadra to the Backbone; a BIND problem.
Lastly, one odd difficulty afflicting switched or routed networks with multiple-IP per MAC (not Mac as in Macintosh) addresses---sometimes the switch/router/hub needs to be physically powered off for some amount of time to clear its switch fabric or routing tables (yes, even if you've switched it off, or flushed the tables, etc...); for cage routers, the suspect card must be pulled, too---sometimes in inexpensive router/switch/hub products, one must pull the RJ45 cables in lieu of pulling a card (but still after pulling the physical power), though I don't know that I've done this more than once, if at all (pulling the cables, not the card or disconnecting physical power---that I've done plenty).
Hope this helps---please post the info if you still need another pair of eyes to look at the problem you've been staring at.
Routing/bridging products include LaserWriter Bridge (shareware, I think, also may be called LocalTalk Bridge), Apple's AppleTalk Router, and Asante's physical bridges (these would simplify your problem in some ways, and complicate it in others). MacDNS can provide you with closed-network name resolution, but I don't remember if it supports the 'forwarders' declaration to get you out to the World.
KAH
The signal point to me seems to be a bit of confusion in your network topology. LocalTalk is an AppleTalk hardware-layer interface; it handles AppleTalk connections across LocalTalk physical media (a 230Kbs paired TX-RX serial line connection terminated at its distant ends). There are many LocalTalk implementation---PhoneNet was very popular and inexpensive, Apple made its own LocalTalk physical media, and so did some others.
EtherTalk is also an AppleTalk hardware-layer interface; it handles AppleTalk connections over Ethernet physical media (10Base-2, 10Base-5, 10Base-T, 100Base-TX, 10/100Base-TX, etc..). There is even a FDDITalk and of course, TokenTalk.
ALL of these are AppleTalk networks, even those that use Ethernet physical media.
From what you describe, it would appear your Quadra is connected to two physically discrete networks employing different software architectures; one AppleTalk network---eMate to Quadra---and one TCP/IP network---Quadra to Backbone.
Unless you have actual Macintosh routing or bridging software, or A/UX with Packet Forwarding enabled, your Quadra will NOT route data between the two Networks.
It would be helpful if you could tell me what information you discover from opening the MacTCP and Network Control Panels---the full IP Address, the full network mask (not '.xxx' please), the full router IP, DNS server info (unless that is publicly accessible, of course, in which case you have another problem), Domain Name info, and any Hosts file info (for MacTCP), and which Interface is selected (in the the Network CP).
DHCP is problematic for small networks, particularly if they connect to wider LANs or WANs (like your ISP). While useful for some applications, it is a lot of trouble for small networks. Unless you have a compelling reason to employ DHCP, don't; it makes Network troubleshooting very difficult unless you have control over every aspect of the network, have all the complimentary software, and lots of bandwidth to burn (which your Quadra certainly does NOT---it can maybe hit 1Mb/s throughput on Ethernet, which is still better than a DOSBOX).
I do not believe MacTCP is your problem. I run MacTCP in a multi-vendor IP environment with no problems---MacTen, OpenTransport, 10.4, AIX 4, A/UX---trouble-free. I suspect you have TWO problems interfering with one another---a confused Quadra, and an attempt to route/bridge from the Quadra to the Backbone; a BIND problem.
Lastly, one odd difficulty afflicting switched or routed networks with multiple-IP per MAC (not Mac as in Macintosh) addresses---sometimes the switch/router/hub needs to be physically powered off for some amount of time to clear its switch fabric or routing tables (yes, even if you've switched it off, or flushed the tables, etc...); for cage routers, the suspect card must be pulled, too---sometimes in inexpensive router/switch/hub products, one must pull the RJ45 cables in lieu of pulling a card (but still after pulling the physical power), though I don't know that I've done this more than once, if at all (pulling the cables, not the card or disconnecting physical power---that I've done plenty).
Hope this helps---please post the info if you still need another pair of eyes to look at the problem you've been staring at.
Routing/bridging products include LaserWriter Bridge (shareware, I think, also may be called LocalTalk Bridge), Apple's AppleTalk Router, and Asante's physical bridges (these would simplify your problem in some ways, and complicate it in others). MacDNS can provide you with closed-network name resolution, but I don't remember if it supports the 'forwarders' declaration to get you out to the World.
KAH