Network services
DHCP
Automates IP assignment, killing config errors and IP conflicts. It hands out four things: IP, subnet mask, default gateway, DNS server. Full walkthrough in my CCNA DHCP note.
A scope is the assignable range on a subnet, with exclusions and reservations (an IP held for a specific device). The lease defaults to 24 hours; corporate networks run longer.
DORA: Discover, Offer, Request, Acknowledge.
DHCP relay forwards client broadcasts to a server on another subnet, so you don’t need a server per subnet. On the router that’s the IP helper address forwarding the UDP broadcast.
If DHCP fails, the fallback is APIPA (default) or a static alternate config.
SLAAC
IPv6 devices configure themselves: generate a link-local address, send a router solicitation, get a router advertisement carrying the prefix, combine prefix + unique identifier, then run a neighbor solicitation to check for duplicates.
No server, no conflicts, less admin overhead.
DNS
Resolves names to IPs. An FQDN sits under a top-level domain, and the hierarchy runs top down: root, TLD (.com, .uk), second-level (diontraining), subdomain (www), host.
The hosts file is checked before DNS: useful for testing, a security risk if tampered with.
Resolution comes in recursive lookups (the resolver does all the chasing) and iterative (each server refers you onward). Resolvers cache answers per the record’s TTL.
Internal DNS serves private networks; external DNS serves public names.
DNS record types
- A: name to IPv4.
- AAAA: name to IPv6.
- CNAME: name to another name (never an IP).
- MX: the mail server for the domain. Lowest priority number wins; equal values load balance.
- SOA: zone info: update timers, admin. Drives zone transfers (TCP).
- PTR: IP to name (reverse lookup, living under in-addr.arpa).
- TXT: arbitrary text: domain verification, email anti-spam.
- NS: the authoritative nameservers (primary and secondary).
Securing DNS
DNSSEC uses cryptographic signatures to prove records weren’t tampered with: integrity, not encryption.
DoH puts DNS inside HTTPS, blending with web traffic. DoT puts DNS inside a TLS tunnel. Both stop DNS snooping (inferring your browsing from queries).
NTP
Syncs clocks over UDP 123 to within milliseconds of UTC. The stratum hierarchy: 0 is reference clocks (atomic), 1 is primary servers attached to stratum 0, 2-15 sit each level further out, and 16 means unsynchronized.
PTP achieves sub-microsecond accuracy (trading floors, industrial automation) with primary and secondary clocks. NTS adds cryptographic security to NTP via TLS and AEAD, authenticating the time source.
Quality of Service
Categorize traffic, prioritize it, allocate bandwidth, and drop low-priority traffic under congestion. Watch three enemies: delay (ms in transit), jitter (uneven arrival, brutal for VoIP), and drops (congestion loss). Effective bandwidth is the slowest link in the path.
The models: best effort (no QoS, FIFO), DiffServ (soft QoS, marked priorities), IntServ (hard QoS, strict reservations).
The mechanisms:
- Classification inspects headers and ports without changing bits; marking sets ToS bits (IP precedence, DSCP).
- Queuing manages congestion: weighted fair, low-latency, weighted round-robin.
- RED avoids congestion by dropping low-priority traffic early.
- Policing discards over-rate packets; shaping buffers and delays them instead.
- Link efficiency: compression (cRTP for VoIP) and LFI, which interleaves small voice packets between big data packets on slow links.