ifconfig and netstat belong to the older net-tools package. Current systems use ip and ss from iproute2.
ip addr show # addresses per interface
ip route # routing table, default gateway
ss -tulpn # listening TCP/UDP ports and their processes
ping -c 3 8.8.8.8 # reachability
dig +short example.com # DNS lookup (package dnsutils)flowchart LR
A[Application] --> B[DNS lookup]
B --> C[Routing table]
C --> D[Default gateway]
D --> E[Remote host]
Ubuntu 18.04 configures networking with Netplan: files in /etc/netplan/*.yaml, applied with sudo netplan apply. CentOS 7 still uses /etc/sysconfig/network-scripts/ifcfg-* files. If a name resolves in dig but not in an application, check /etc/resolv.conf and /etc/nsswitch.conf.