Domain 1 — Networking Fundamentals

Week of 2026-09-15 · Download .docx

Objectives

Explain the OSI model and TCP/IP model layer functions and PDUs.
Compare network topologies including bus, ring, star, mesh, and three-tier enterprise.
Apply IPv4 subnetting using CIDR notation, VLSM, and private address ranges.
Compare IPv4 and IPv6 addressing, abbreviation rules, and transition mechanisms.
Identify well-known port numbers and explain TCP vs. UDP and the DHCP DORA process.

Key terms

OSI model: Seven-layer reference model: Physical, Data Link, Network, Transport, Session, Presentation, Application.
PDU: Protocol Data Unit — the data unit name at each OSI layer: bit (L1), frame (L2), packet (L3), segment (L4).
TCP/IP model: Four-layer practical model: Network Access, Internet, Transport, Application.
three-tier architecture: Core (backbone speed), Distribution (routing/policy), Access (end devices) — the enterprise LAN design standard.
full mesh: Every device connects to every other device; n*(n-1)/2 links required.
CIDR: Classless Inter-Domain Routing — slash notation (e.g., /24) for subnet prefix length.
VLSM: Variable Length Subnet Masking — assigning different-sized subnets from one block to conserve IP space.
APIPA: Automatic Private IP Addressing — 169.254.0.0/16 self-assigned when DHCP is unreachable.
dual-stack: Running IPv4 and IPv6 simultaneously on the same device.
SLAAC: Stateless Address Autoconfiguration — IPv6 hosts assign their own link-local address without a DHCP server.
anycast: One IP assigned to multiple devices; the routing protocol delivers packets to the nearest one.
DORA: DHCP four-step process: Discover, Offer, Request, Acknowledge.
3-way handshake: TCP connection setup: SYN from client, SYN-ACK from server, ACK from client.

The concept

THE OSI AND TCP/IP MODELS

Every network conversation follows a layered model. The OSI model defines seven layers, each responsible for one category of function: Physical (bits on the wire), Data Link (frames and MAC addresses), Network (IP packets and routing), Transport (segments/datagrams and reliability), Session (dialog management), Presentation (encoding), and Application (user-facing protocols like HTTP). The TCP/IP model condenses this to four practical layers used in real implementations: Network Access (combining OSI L1+L2), Internet (OSI L3), Transport (OSI L4), and Application (OSI L5–7). When studying for the exam, match each protocol to its OSI layer — HTTP is Layer 7, TCP/UDP is Layer 4, IP is Layer 3, Ethernet is Layer 2, the cable is Layer 1.

NETWORK TOPOLOGIES AND THREE-TIER DESIGN

Topology describes the physical or logical arrangement of devices. A star topology (the modern standard) connects all devices to a central switch. A full mesh connects every device to every other, requiring n*(n-1)/2 links and providing maximum redundancy. Enterprise networks use the three-tier model: the Core provides high-speed backbone switching, the Distribution layer enforces routing and access policy between VLANs, and the Access layer connects end devices. Understanding which tier handles which function is frequently tested.

IPv4 ADDRESSING AND SUBNETTING

IPv4 addresses are 32 bits, written as four decimal octets. The three RFC 1918 private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. CIDR notation specifies how many bits are the network prefix; the remaining bits are the host portion. For a /26 subnet, there are 6 host bits (64 total addresses, 62 usable after removing network and broadcast). VLSM allows different-sized subnets from one address block — a /30 for point-to-point links wastes only 2 addresses. NAT (and PAT) allows many private addresses to share one public IP, conserving the global IPv4 space.

IPv6 AND TRANSITION MECHANISMS

IPv6 addresses are 128 bits in eight hex groups separated by colons. Two compression rules apply: remove leading zeros within each group, and replace one run of consecutive all-zero groups with :: (only once per address). IPv6 types are unicast (one host), multicast (group, ff02::/16), and anycast (nearest of many). There is no broadcast. Every interface automatically receives a link-local address (fe80::/10) via SLAAC — no DHCP needed for local communication. Dual-stack runs both protocol stacks simultaneously. 6to4 tunneling carries IPv6 inside IPv4 packets across legacy networks.

PORTS, PROTOCOLS, AND SERVICES

Well-known ports (0–1023) must be memorized: FTP=20/21, SSH=22, Telnet=23, SMTP=25, DNS=53, HTTP=80, HTTPS=443, IMAP=143, LDAP=389, SNMP=161, RDP=3389, NTP=123. TCP is connection-oriented (3-way handshake before data flows) and guarantees delivery. UDP is connectionless and faster — used for DNS, NTP, SNMP, VoIP. DHCP uses UDP for the DORA handshake: the client broadcasts a Discover, the server Offers an IP, the client Requests it, and the server Acknowledges. Telnet must be replaced by SSH because Telnet sends all data including passwords in cleartext.

Standards alignment: CompTIA Network+ Objective 1.1 (Topologies), 1.2 (IP Addressing); Maryland Blueprint College and Career Readiness pillar — Technology and Engineering.

Worked examples

Example 1: A network has the address block 192.168.50.0/24. You need subnets for: Marketing (60 hosts), IT (14 hosts), two router links (2 hosts each). Using VLSM: assign 192.168.50.0/26 to Marketing (62 usable), 192.168.50.64/28 to IT (14 usable), 192.168.50.80/30 and 192.168.50.84/30 for the two router links (2 usable each). Total addresses used: 64+16+4+4=88 out of 256 — far more efficient than four /24 subnets.

Example 2: A technician runs 'ipconfig' on a laptop and sees the IP address 169.254.22.5. She recognizes this as APIPA — the laptop could not reach the DHCP server. She checks the switch port (the link light is off), replaces the patch cable, and the link comes up. She runs 'ipconfig /release' then 'ipconfig /renew' and the laptop receives 192.168.1.105 from DHCP. Correct diagnosis: Layer 1 cable fault caused DHCP failure.

Common mistakes

Confusing the broadcast address with the last usable host — for a /26, the broadcast is x.x.x.63 and the last usable host is x.x.x.62.
Using :: more than once in an IPv6 address — the double-colon can only appear once to avoid ambiguity.
Mixing up SMTP (25, send) with IMAP (143, retrieve) — remember: SMTP sends, IMAP/POP3 receive.
Assuming APIPA means the NIC is broken — it almost always means DHCP is unreachable, which is usually a cable or server issue.
Placing the Distribution layer function (policy/routing) at the Access layer — access ports connect end devices; policy is enforced at Distribution.

Self-check

Try each question before reading the answer. Answers at the bottom of this page.

1. What is the PDU name at OSI Layer 2?

Segment
Packet
Frame
Bit

2. How many usable host addresses does a /28 subnet provide?

3. What does APIPA address 169.254.x.x indicate?

Static IP was misconfigured
DHCP server was unreachable
DNS server failed
Router is down

4. Which IPv6 address type is automatically assigned to every interface without a DHCP server?

Global unicast
Unique local
Link-local (fe80::/10)
Multicast

5. What is the correct order of the DHCP DORA handshake?

Discover, Offer, Request, Acknowledge
Discover, Request, Offer, Acknowledge
Offer, Discover, Request, Acknowledge
Request, Discover, Offer, Acknowledge

Self-check answers

1. C — Layer 2 (Data Link) PDUs are frames — they contain MAC addresses and error-check fields.
2. B — A /28 has 4 host bits: 2⁴=16 total addresses minus network and broadcast = 14 usable hosts.
3. B — APIPA self-assigns when the DHCP server cannot be reached within the timeout window.
4. C — Link-local addresses (fe80::/10) are auto-configured via SLAAC on every IPv6 interface.
5. A — DORA: the client Discovers, the server Offers, the client Requests confirmation, the server Acknowledges.

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