Security issues may occur on all site, i.e client, network and server sites.
Servers may face security attacks if connect to internet , therefore suitable firewall is recommended for safety.
For clients, viruses attacks are common because users may use USB's flash drives very frequently, therefore a centralized anti-virus is recommended.
while security issue from the network only when two or more than two LAN connected with each other in other words WAN, MAN is using.