DNS, DHCP, NTP, Active Directory, LDAP, RADIUS, TACACS+, and PKI — the background services every device and application on the network depends on constantly, and the dependency chain between them that causes the network's most confusing outages.
Earlier modules covered how packets move. This module covers the services that have to answer correctly before those packets can move at all: what address does this name resolve to, what IP address should this device use, what time is it right now, is this user who they claim to be, and is this device or certificate actually trustworthy. Each service is simple in isolation — the real skill is understanding how they depend on each other.
By the end of this module you should be able to explain why a missing IP helper address leaves a brand-new VLAN handing out no addresses at all, why clock drift can silently lock a device out of Active Directory with an error that never mentions time, and when to use RADIUS versus TACACS+ for two authentication jobs that are easy to conflate but solve different problems.