Network+ Domain 1 - Networking Concepts (23%)¶

The vocabulary domain. Nearly everything later in the exam assumes the OSI model, ports, and subnetting are automatic for you.

The OSI model¶

• Encapsulation - each layer wraps the layer above with its own header as data moves down the stack.
• Decapsulation - each header is stripped in turn as data moves up the stack on the receiving host.
• Maximum transmission unit (MTU) - the largest frame payload a link will carry, 1500 bytes on standard Ethernet.
• Jumbo frame - an Ethernet frame with an MTU around 9000 bytes, used in storage and data-center networks to cut per-packet overhead.

Troubleshooting shortcut: a device that makes decisions using MAC addresses is Layer 2, and one using IP addresses is Layer 3. That single distinction answers many questions.

Ports and protocols¶

Learn these cold. They appear throughout the exam.

• TCP (Transmission Control Protocol) - connection-oriented, reliable, ordered. Uses a three-way handshake: SYN, SYN-ACK, ACK.
• UDP (User Datagram Protocol) - connectionless, no delivery guarantee, lower overhead. Preferred for voice, video, and DNS queries.
• ICMP (Internet Control Message Protocol) - error and diagnostic messaging. Carries ping and traceroute, and has no ports.

IP addressing¶

• IPv4 address - 32 bits, written as four dotted octets.
• IPv6 address - 128 bits, written as eight hextets in hexadecimal, with :: compressing one run of consecutive zero groups.
• Subnet mask - marks which bits are network and which are host.
• CIDR notation - the count of network bits, for example /24.
• Default gateway - the router a host sends traffic to when the destination is outside its own subnet.

Private ranges (RFC 1918) - not routable on the internet:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

Also know 169.254.0.0/16 (APIPA, self-assigned when DHCP fails - a very common exam symptom) and 127.0.0.0/8 (loopback).

• NAT (Network Address Translation) - rewrites addresses so private hosts can reach the internet through a public address.
• PAT (Port Address Translation) - many private hosts share one public address, distinguished by port. This is what most home routers do.

IPv6 specifics

• Global unicast - publicly routable, typically 2000::/3.
• Link-local - fe80::/10, automatically configured, valid only on the local link.
• SLAAC (Stateless Address Autoconfiguration) - a host builds its own address from the router advertisement.
• Dual stack - running IPv4 and IPv6 simultaneously. IPv6 has no broadcast; it uses multicast and anycast instead.

Subnetting¶

The most practiced skill in the exam. Memorize the mask table:

Usable hosts is 2^h - 2, subtracting the network and broadcast addresses. A /30 gives two usable addresses, which is why it suits point-to-point links.

• VLSM (Variable Length Subnet Masking) - using different mask lengths within one network so subnet sizes fit actual need.
• Supernetting / route summarization - combining contiguous networks into one advertisement to shrink routing tables.

Topologies and network types¶

• Star - every node connects to a central device. Dominant in modern LANs.
• Mesh - nodes interconnect directly; full mesh gives maximum redundancy at high cost.
• Hybrid - a mixture, which is what real networks are.
• Point-to-point - two endpoints, typically a WAN link.
• LAN, WAN, MAN, PAN, CAN, SAN, WLAN - scope descriptors from a single room to a metropolitan area. A SAN is a storage-specific network.

Cloud concepts¶

• IaaS - you manage the OS upward.
• PaaS - you manage the application and data.
• SaaS - you manage configuration and data only.

See IaaS, PaaS, SaaS explained and the shared responsibility model.

• Public, private, hybrid, and community cloud - deployment models distinguished by who owns the infrastructure and who may use it.
• VPC (Virtual Private Cloud) - a logically isolated network inside a provider's cloud.
• Direct Connect / ExpressRoute - dedicated private circuits between on-premises and the cloud, bypassing the internet.
• NFV (Network Function Virtualization) - running firewalls, load balancers, and routers as software instances.

Network services¶

• DHCP - automatic IP configuration. The DORA sequence is Discover, Offer, Request, Acknowledge.
• DHCP relay / IP helper - forwards DHCP broadcasts across a router to a server on another subnet.
• DNS - resolves names to addresses. Record types: A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), TXT (arbitrary text, used for SPF), PTR (reverse), NS (name server), SOA (zone authority).
• NTP - synchronizes clocks. Stratum numbers indicate distance from the reference clock.
• SNMP - device monitoring. v3 adds authentication and encryption; v1 and v2c send community strings in clear text.

See DNS explained for the resolution walk-through.

Network devices¶

• Hub - Layer 1, repeats to all ports, one collision domain. Obsolete.
• Switch - Layer 2, forwards by MAC address, each port its own collision domain.
• Router - Layer 3, forwards between networks by IP, separates broadcast domains.
• Firewall - filters traffic by rule. Stateful firewalls track connection state.
• Load balancer - distributes traffic across servers. See load balancing deep dive.
• Proxy - forward proxies act for clients, reverse proxies act for servers.
• IDS / IPS - an IDS detects and alerts; an IPS sits inline and blocks.
• Wireless controller - centrally manages access points.
• Content delivery network (CDN) - geographically distributed caching. See CDN explained.

Exam pointers¶

• A host with a 169.254.x.x address did not reach a DHCP server. This is one of the most common scenario answers on the exam.
• Duplicate IP addresses cause intermittent connectivity for both hosts.
• If a question distinguishes collision domains from broadcast domains: switches split collision domains, routers split broadcast domains.
• Practice subnetting until block size arithmetic is instant. It is the highest-yield preparation you can do.

Official documentation¶

📖 CompTIA Network+ N10-009 objectives - authoritative domain list 📖 RFC 1918 - private address allocation 📖 IANA port number registry - authoritative port assignments