Networking problem on Ubuntu 18.04: no gateway

Question

After a fresh installation of Ubuntu 18.04 LTS I have configured netplan to use a static IP address by following the instructions available online here:

https://www.tecmint.com/ifconfig-vs-ip-command-comparing-network-configuration/

The problem is that the gateway IP address seems to be ignored. This is the yaml file (/etc/netplan/50-cloud-init.yaml):

network:
version: 2
renderer: networkd
ethernets:
    ens160:
        dhcp4: no
        dhcp6: no
        addresses: [172.20.0.109/24]
        gateway4: 172.20.3.254
        nameservers:
            addresses: [172.20.0.5,172.20.0.6]

This the output of networkctl status ens160 (after executing sudo netplan apply) where you can see that the gateway is missing:

● 2: ens160
   Link File: /lib/systemd/network/99-default.link
Network File: /run/systemd/network/10-netplan-ens160.network
        Type: ether
       State: routable (configured)
        Path: pci-0000:03:00.0
      Driver: vmxnet3
      Vendor: VMware
       Model: VMXNET3 Ethernet Controller
  HW Address: 00:50:56:a9:bc:e5 (VMware, Inc.)
     Address: 172.20.0.109
              fe80::250:56ff:fea9:bce5
         DNS: 172.20.0.5
              172.20.0.6

This is the file Network File (/run/systemd/network/10-netplan-ens160.network):

[Match]
Name=ens160

[Network]
Address=172.20.0.109/24
Gateway=172.20.3.254
DNS=172.20.0.5
DNS=172.20.0.6

This is the output of route -n:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
172.20.0.0      0.0.0.0         255.255.255.0   U     0      0        0 ens160

Solution

You have a misconfigured network configuration because the gateway IP address is out of the current network (/24).

Assuming that the gateway ip address is correct, you probably need to configure a /22 instead of /24 network to be able to reach the gateway.

Check HostMin and HostMax with ipcalc utility to see how netmask affect network reachability.

$ ipcalc 172.20.0.109/24
Address:   172.20.0.109         10101100.00010100.00000000. 01101101
Netmask:   255.255.255.0 = 24   11111111.11111111.11111111. 00000000
Wildcard:  0.0.0.255            00000000.00000000.00000000. 11111111
=>
Network:   172.20.0.0/24        10101100.00010100.00000000. 00000000
HostMin:   172.20.0.1           10101100.00010100.00000000. 00000001
HostMax:   172.20.0.254         10101100.00010100.00000000. 11111110
Broadcast: 172.20.0.255         10101100.00010100.00000000. 11111111
Hosts/Net: 254                   Class B, Private Internet


$ ipcalc 172.20.0.109/22
Address:   172.20.0.109         10101100.00010100.000000 00.01101101
Netmask:   255.255.252.0 = 22   11111111.11111111.111111 00.00000000
Wildcard:  0.0.3.255            00000000.00000000.000000 11.11111111
=>
Network:   172.20.0.0/22        10101100.00010100.000000 00.00000000
HostMin:   172.20.0.1           10101100.00010100.000000 00.00000001
HostMax:   172.20.3.254         10101100.00010100.000000 11.11111110
Broadcast: 172.20.3.255         10101100.00010100.000000 11.11111111
Hosts/Net: 1022                  Class B, Private Internet

Comments

[Davide Merlitti]

Thanks a lot Giovanni! With netmask 22 now I reach the gateway.