A+ Core 1 Networking: IP Addressing, DHCP, DNS, and Wi-Fi

Read the address before you change anything

Networking is 23% of the 220-1201 blueprint. Its objectives cover ports, Wi-Fi, network services, DNS and DHCP configuration, hardware, SOHO addressing, connection types, and tools. Ports have their own page, A+ ports and protocols. This page covers the addressing, DHCP, DNS, and Wi-Fi decisions that scenario questions build on.

The fastest diagnosis usually comes from the client's own settings, for example from ipconfig /all on Windows. Learn to read them for clues:

The 169.254 clue comes from RFC 3927, which reserves 169.254/16 for IPv4 link-local addresses. Routers must not forward packets with those addresses. That is why an APIPA client may still reach another self-assigned device on the same link, but never the internet. Microsoft's ipconfig reference also notes that the output tells you whether settings came from DHCP, APIPA, or an alternate configuration.

IPv4 and IPv6 basics the exam expects

Private vs public. RFC 1918 sets aside three IPv4 blocks for private networks: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. A SOHO router typically hands out private addresses inside and shares one public address toward the internet. An address such as 172.32.1.10 is outside 172.16.0.0/12, so it is not private. That is a favorite trap.

Subnet mask and gateway. With 192.168.20.0 and a mask of 255.255.255.0 (/24), the hosts are 192.168.20.1 through .254. The default gateway must be one of those addresses. A gateway of 192.168.2.1 on that client is a typo, not a design choice.

Static vs dynamic. Clients normally get addresses dynamically. Printers, servers, and network gear need addresses that do not move, either by static configuration or by a DHCP reservation, explained below.

IPv6. IPv6 addresses are 128 bits, written in hexadecimal. RFC 4291 defines ::1 as the loopback address and fe80::/10 as the link-local prefix. Link-local addresses are for use on a single link, and routers must not forward them.

DHCP: scopes, exclusions, reservations, and leases

The A+ objectives list four DHCP terms. Microsoft's Windows Server documentation defines the first three, and RFC 2131 sets the lease timers:

• Scope. The range of addresses a DHCP server can lease to clients on one subnet, plus options such as the gateway and DNS servers.
• Exclusion. Addresses inside the scope that the server will not hand out. Use exclusions for anything configured statically, so DHCP never creates a conflict.
• Reservation. A specific address that the server always gives to one device, matched by its MAC address. The device still uses DHCP. It just always receives the same answer.
• Lease. The address is loaned for a set time. RFC 2131 sets the default renewal timer, T1, at half the lease, and the rebinding timer, T2, at 87.5% of the lease.

A small-office example. A fictional office uses 192.168.50.0/24. The DHCP scope covers .1 through .254. The router (.1), the NAS (.2), and room to grow are excluded through .20. The front-desk printer gets a reservation at .60, so its address never changes, but it still pulls DNS settings from DHCP. A laptop gets .113 with an eight-hour lease and tries to renew after about four hours.

A client's first request follows the order RFC 2131 describes: the client broadcasts a discover, servers offer, the client requests one offer, and the chosen server acknowledges it. If nothing answers and no alternate configuration is set, Windows falls back to the 169.254 address from the table above.

DNS records you need to recognize

The three spam-management records work together, and each lives in a TXT record:

• SPF lists which hosts may send mail using the domain in the envelope sender and HELO identities. RFC 7208 says SPF records must be published as DNS TXT records.
• DKIM signs each message. The signature travels in a DKIM-Signature header, and the receiver fetches the public key from DNS under a selector in the _domainkey subdomain, for example selector1._domainkey.example.com.
• DMARC tells receivers what to do when mail fails. It is published as a TXT record at _dmarc.example.com, with a policy of none, quarantine, or reject. A DMARC pass needs an SPF or DKIM pass for a domain that aligns with the visible From domain. DMARC's current specification is RFC 9989, published in May 2026, which replaced RFC 7489.

A typical A+ scenario: a company's invoices land in customers' spam folders after it moves to a new mail provider. The first suspect is an SPF record that still lists only the old provider's servers.

VLANs and VPNs sit in the same objective. A VLAN splits one switched network into separate logical networks, such as staff and guests. A VPN carries traffic through an encrypted tunnel across a network you do not trust.

Wi-Fi bands and generations

The 2.4 and 5 GHz trade-offs and the 1, 6, and 11 channel advice come from Microsoft's Wi-Fi support guidance. The A+ objectives list "802.11 standards" without detail, so learn each standard with its band and, for newer ones, the generation name the Wi-Fi Alliance uses. Bands for 802.11a through 802.11ax follow Intel's protocol reference.

A help-desk version of the decision: a user far from the access point, behind two walls, may do better on 2.4 GHz. A user in the same room who streams video should be on 5 GHz, or on 6 GHz if both the laptop and the access point support it.

Tools named in the networking objectives

When a stem describes a symptom, the clue table at the top of this page is the place to start. The hardware and network troubleshooting guide covers network symptoms such as jitter and port flapping. The Windows command-line guide shows ipconfig, ping, and nslookup in use. To test yourself, practice A+ Networking domain questions with explanations in Passy →, or try the free A+ practice set.