Networking
Security and internet connection, network administration (VLAN, spanning-tree, routing, firewall, multicast ...), proxy/reverse-proxy, web filtering, Cisco ASA, Fortinet Fortigate, Sophos, network monitoring (Nagios, Centreon, Zabbix)
Computing
Linux management (bash, virtualization, container, high availability, network services, web services, LDAP ...), Windows Server management (PowerShell, Hyper-V, Active Directory, GPO ...), cloud (Terraform, Ansible), web development (PHP, HTML, CSS, JS), database (SQL, NoSQL), mobile app development (Cordova), Node JS, programming (C, Python)
Telecoms
Corporate telephony PABX (Astraa) and IPBX (Asterisk), VOIP security and ciphering, mobile telephony, digital transmissions, telecom electronics