Some of the typical requirements for a Enterprise-level LAN are:

• Usually multiple buildings are involved;
• Data transfers between departments, sites, associated companies. clients and backup solutions;
• High speed data connectivity;
• A large number of people requiring access, typically with desktops and/or laptops;
• Access by smartphones/tablets may be required, restricted to authorised devices only;
• Possibly limited public wireless connectivity (for visitors, clients, etc);
• Full PABX functionality, often including in-dial telephone ranges and IVRs;
• Staff require access from sites outside the company buildings (and may even work-from-home).

In this scenario, your network would comprise of a multiple firewalls, an internal routed network connecting all your buildings, switched networks within each building, dedicated large-scale VoIP PABX/s and other servers. Wireless connectivity may be provided using multiple separate SSIDs (typically one with appropriate security for appropriate staff devices and one with limited security for visitors/guests).

The firewalls and routers will be enterprise-level business-grade models (eg: Appropriate large scale Cisco router). The DMZ would hold all internet accessible servers (such as web gateway server, email gateway server, VoIP gateway server, VPN server, etc). Each building/department may have their own servers, which they may administer, but there is co-ordination at a company level to ensure there are no conflicts (such as telephone extension number ranges, IP address range allocations, etc).