Hi Pedro, Thanks for filing the bug and making cloud-init better.

I can confirm locally with the following:
cat > LP1978543.yaml <<EOF
version: 2
ethernets:
  enp0s3:
    dhcp4: false
    addresses: [10.0.4.10/24]
    nameservers:
      addresses: [10.0.4.1]
    routes:
    - to: default
      via: 10.0.4.1
      metric: 100
EOF

PYTHONPATH=. python3 -m cloudinit.cmd.main devel net-convert -m enp0s3,AA -O netplan --k yaml -p LP1978543.yaml  -D ubuntu -d out
Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/csmith/src/cloud-init/cloudinit/cmd/main.py", line 1073, in <module>
    return_value = main(sys.argv)
  File "/home/csmith/src/cloud-init/cloudinit/cmd/main.py", line 1059, in main
    retval = util.log_time(
  File "/home/csmith/src/cloud-init/cloudinit/util.py", line 2621, in log_time
    ret = func(*args, **kwargs)
  File "/home/csmith/src/cloud-init/cloudinit/cmd/devel/net_convert.py", line 136, in handle_args
    ns = network_state.parse_net_config_data(pre_ns)
  File "/home/csmith/src/cloud-init/cloudinit/net/network_state.py", line 1056, in parse_net_config_data
    nsi.parse_config(skip_broken=skip_broken)
  File "/home/csmith/src/cloud-init/cloudinit/net/network_state.py", line 279, in parse_config
    self.parse_config_v2(skip_broken=skip_broken)
  File "/home/csmith/src/cloud-init/cloudinit/net/network_state.py", line 328, in parse_config_v2
    handler(self, command)
  File "/home/csmith/src/cloud-init/cloudinit/net/network_state.py", line 721, in handle_ethernets
    subnets = self._v2_to_v1_ipcfg(cfg)
  File "/home/csmith/src/cloud-init/cloudinit/net/network_state.py", line 880, in _v2_to_v1_ipcfg
    _normalize_route(
  File "/home/csmith/src/cloud-init/cloudinit/net/network_state.py", line 1019, in _normalize_route
    _normalize_net_keys(
  File "/home/csmith/src/cloud-init/cloudinit/net/network_state.py", line 953, in _normalize_net_keys
    raise ValueError(f"Address {addr} is not a valid ip address")
ValueError: Address default is not a valid ip address


When I change `to: default` to the alternative `to: 0.0.0.0/0` we render correctly without error 
csmith@uptown:~/src/cloud-init$ PYTHONPATH=. python3 -m cloudinit.cmd.main devel net-convert -m enp0s3,AA -O netplan --k yaml -p LP1978543.yaml  -D ubuntu -d out
Read input format 'yaml' from 'LP1978543.yaml'.
Wrote output format 'netplan' to 'out/'

csmith@uptown:~/src/cloud-init$ cat out/etc/netplan/50-cloud-init.yaml 
# This file is generated from information provided by the datasource.  Changes
# to it will not persist across an instance reboot.  To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    ethernets:
        enp0s3:
            addresses:
            - 10.0.4.10/24
            dhcp4: false
            nameservers:
                addresses:
                - 10.0.4.1
            routes:
            -   metric: 100
                to: 0.0.0.0/0
                via: 10.0.4.1
    version: 2