If you lack an Ethernet pseudo-device like tap or bpf, it is still possible to network the machine via the serial port using the PPP protocol. You must configure one of the serial ports; on Sun machines, these are usually enumerated as ttya or ttyb, depending on which one is configured in your machine description. It also must not be connected to keyboard, mouse, or console of course. In this case, we simulate a NULL modem via the same mechanism used for the console. You must configure a serial port to use a posix serial device. See the sample line for the console, for example. Afterwards, both host and guest must run a PPP program that will allow them to talk over the serial line. Most Unix-y platforms have a PPPD or equivalent; see your system documentation/packages for details.
For example, using a NetBSD guest on a Linux host, you can use the same pppd package on both, with corresponding options. They both offer the same options, so you can use, e.g.:
pppd nodetach noauth local /dev/pts/2 38400
on the host (replacing pts/2 with whatever slave device is used by TME) and:
pppd nodetach noauth local /dev/ttya 38400
on the guest (using ttya or ttyb depending on which is configured for your guest machine).
Note that 38400 is the max speed supported by Sun machines (ref?). This configures static IP addresses of 192.168.1.2 on the host and 192.168.1.1 on the guest. Alternatively, you may omit that addresses and get them assigned automatically. If you have an interface to another LAN on the host machine, you can even use Proxy ARP to forward packets from machines on the LAN to this machine and back by using an available address from the LAN subnet. Besides NAT, this is another way to get on an external network pretty quickly. On FreeBSD, there is a user-space PPP program that will set up NAT to an external network as well. See the next section for more info about NAT.
It should be possible to set up a P2P link between two machines by following the above steps on both guests and then using, e.g., socat to set up a relay between the machines to simulate a NULL modem. I haven’t successfully done this yet, but I see no reason why it can’t be done. In fact, socat can be used for general packet relay and is generally useful for non-standard network setups where other options are not available. See the socat docs for details.