Network Services

What this unit covers

The topics below follow the published Networking course framework for Unit 3. Networking publishes no per-unit weighting, so there is no percentage to chase here.

Lessons in this unit

• DNS, DHCP, and Address Translation18 min · 3 objectivesExplain core network services · Apply the idea to evidence from a realistic technology scenario · Justify a decision using security, reliability, cost, and user impact
• Transport and Application Services18 min · 3 objectivesExplain TCP, UDP, ports, and application dependencies · Apply the idea to evidence from a realistic technology scenario · Justify a decision using security, reliability, cost, and user impact
• Cloud, Virtualization, and Service Resilience18 min · 3 objectivesExplain virtual networks, redundancy, and shared responsibility · Apply the idea to evidence from a realistic technology scenario · Justify a decision using security, reliability, cost, and user impact

Every term in Unit 3

All 40 terms we publish for Network Services, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

DHCP

Automatically assigns addressing to clients: IP address, subnet mask, default gateway and DNS servers. Without it every device is configured by hand.

DORA

Discover, Offer, Request, Acknowledge — the four DHCP messages, in that order. The client broadcasts first because it has no address yet.

DNS

Resolves human-readable names to IP addresses. When DNS fails, everything appears broken even though the network is fine — which is why it is the first thing to test.

DNS resolution order

A client checks its cache, then the hosts file, then its configured resolver, which queries the hierarchy from root to top-level domain to authoritative server.

A and AAAA records

A maps a name to an IPv4 address; AAAA maps it to an IPv6 address. Same job, different address family.

CNAME record

An alias pointing one name at another name, which is then resolved. Used so several services can follow one canonical host.

MX record

Names the mail servers for a domain, with a preference value setting the order they are tried.

PTR record

Reverse lookup, mapping an address back to a name. Used by mail servers and logging to check that a host is what it claims.

TXT record

Arbitrary text on a domain, used in practice for SPF, DKIM and domain ownership verification.

NS record

Names the authoritative name servers for a zone — how delegation from a parent zone works.

Time to live (TTL)

How long a resolver may cache a record. Lowering it before a planned change is why migrations do not strand users on the old address.

Authoritative vs recursive DNS server

An authoritative server holds the real records for a zone. A recursive resolver answers clients by asking others and caching the result.

DHCP lease

A time-limited assignment that the client renews. Expiry lets addresses return to the pool when devices leave.

DHCP reservation

Binding a specific address to a specific MAC, so a device always gets the same one while still using DHCP for the rest of its configuration.

DHCP relay

Forward DHCP broadcasts to a server on another subnet, since broadcasts do not cross routers. Why one server can serve many VLANs.

HTTP and HTTPS

The protocols carrying web traffic — port 80 and 443. HTTPS is HTTP inside TLS, so an observer sees the host but not the path or content.

FTP, SFTP and FTPS

FTP transfers files in the clear. SFTP runs over SSH and FTPS wraps FTP in TLS; both encrypt, and they are different protocols despite the similar names.

SMTP, POP3 and IMAP

SMTP sends mail between servers; POP3 downloads and typically removes it; IMAP keeps mail on the server and syncs across devices.

SSH

Encrypted remote command-line access and secure tunneling, on port 22. The replacement for Telnet, which sends everything including the password in plain text.

RDP

Remote Desktop Protocol, port 3389, for graphical remote access to Windows. Frequently attacked when exposed to the internet.

NTP

Synchronizes clocks across a network. Matters more than it sounds: certificate validation, authentication tickets and log correlation all break when clocks disagree.

LDAP and directory services

A protocol for querying a directory of users, groups and devices — the backbone of centralized authentication and permissions.

SNMP

Collects status and metrics from network devices, and can send unsolicited traps on events. Version 3 adds authentication and encryption the earlier versions lack.

Syslog

A standard for shipping log messages to a central server, with severity levels. Centralized logs survive the failure of the device that produced them.

Port number

A 16-bit number identifying a service on a host. The IP address finds the machine; the port finds the process on it.

Well-known ports 20 and 21

FTP data and FTP control.

Well-known ports 22, 23, 25

SSH, Telnet and SMTP. Telnet and SMTP without encryption are both plain text.

Well-known ports 53, 67, 68

DNS, and DHCP server and client.

Well-known ports 80, 110, 143

HTTP, POP3 and IMAP.

Well-known ports 161, 389, 443, 3389

SNMP, LDAP, HTTPS and RDP.

Socket

The pairing of an IP address and a port number. A connection is identified by the pair of sockets at its two ends.

Client-server model

A client requests, a server provides. The same machine can be both for different transactions, which is why the terms describe roles rather than hardware.

Peer-to-peer model

Every node acts as both client and server. Resilient and hard to administer or secure centrally.

Proxy server

Makes requests on a client's behalf, allowing caching, filtering and logging, and hiding the client from the destination.

Reverse proxy

Sits in front of servers, taking client requests and distributing them. Provides TLS termination, caching and a single public entry point.

Content delivery network

Distributed caching servers placing content near users. Cuts latency and absorbs load, including some denial-of-service traffic.

Web server vs application server

A web server delivers content over HTTP; an application server runs the program that generates it. Often deployed together and separated for scale and security.

Certificate and TLS handshake

The server presents a certificate proving its identity, the two sides agree a session key, and traffic is then symmetrically encrypted. The padlock means this succeeded — nothing more.

Service dependency

Most services fail if DNS, DHCP, NTP or directory services fail. Diagnosing an outage means checking these before the application itself.

Port scanning a service inventory

Knowing which ports are open on your own hosts is basic hygiene: every open port is a service, and every unnecessary service is avoidable exposure.

What examiners penalize here

• For a scenario about **core network services**, identify the decisive evidence before naming a response. A defensible conclusion here is: Investigate DHCP and local connectivity before blaming name resolution.
• For a scenario about **TCP, UDP, ports, and application dependencies**, identify the decisive evidence before naming a response. A defensible conclusion here is: Different parts of one application can choose different transports based on their requirements.
• For a scenario about **virtual networks, redundancy, and shared responsibility**, identify the decisive evidence before naming a response. A defensible conclusion here is: Multiple front-end servers do not make the whole service resilient when a critical shared dependency is singular.

Practice Networking

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 5 units of AP Networking

1. Unit 1 · Network Foundations
2. Unit 2 · Network Infrastructure
3. Unit 3 · Network Services
4. Unit 4 · Network Operations
5. Unit 5 · Network Security