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6.6 Describe the functionality of DHCP, DNS, NAT, SNMP, NTP

DHCP

  • dynamically assigns network config (IP, subnet mask, gateway, DNS) to hosts, replacing error-prone manual configuration.
  • Uses client/server model over UDP: servers listen on port 67, clients on port 68.
  • Supports "options" for extra config data (e.g., option 150 for TFTP servers on Cisco IP Phones, option 43 for wireless LAN controllers).
  • Three allocation methods:
    • automatic (permanent IP)
    • dynamic (leased IP for a set time)
    • manual (admin-assigned, relayed via DHCP).
  • Key benefits: centralized management and reduced configuration time/cost vs. manual setup.
  • When client and server are on different subnets, a DHCP relay agent (usually the default router) forwards messages between them.
  • Process follows four phases, often called DORA:
    1. Discover – client broadcasts DHCPDISCOVER (source 0.0.0.0, dest 255.255.255.255)
    2. Offer – server responds with DHCPOFFER (includes chaddr, yiaddr, and options)
    3. Request – client broadcasts DHCPREQUEST accepting one offer
    4. Acknowledgment – server confirms with DHCPACK, binding the client to its IP
  • Clients can voluntarily release their lease early via a DHCPRELEASE message.

SNMP

  • simple network management protocol
  • used for monitoring and managing devices on network
  • UDP 161/162 on application layer 7

Versions

  • v1 – original, rarely used now.
  • v2/v2c – added 64-bit counters, more operations; v2c still widely used (community-string based).
  • v3 – adds security: authentication, encryption, message integrity. This is the "correct" answer whenever a question is about secure SNMP.\

Three components

  • Managed device – the thing being monitored.
  • SNMP agent – software on the device, translates local info into SNMP format.
  • SNMP manager (NMS) – the app/server polling and controlling devices.

MIB

  • Management Information Base, a tree-structured database of manageable objects.

NTP

  • network time protocol
  • Client/server model over UDP port 123 layer 4
    • clients poll servers to stay in sync
  • allows a device to update its clock from a trusted network time source and serve time to other devices
  • Synchronizes time across devices on a network. Critical for SLAs, logging/troubleshooting correlation, and security (e.g., TLS/HTTPS handshakes fail if time is too far off).

Stratum

  • Servers are ranked by "distance" from an authoritative clock
    • stratum 1: atomic/radio clock
    • stratum 2: synced from stratum 1
    • Lower stratum = more trusted.

Security

  • Use encrypted authentication + restrict to trusted servers via ACLs