Chapter 6 — Introduction to the Internet Protocol | Network+ Study Map

CompTIA Network+ · N10-009 · Objectives 1.1 & 1.4

Chapter 6 — Introduction to TCP/IP Protocols and Network Services

Modern networks rely on the TCP/IP protocol suite to define how data is prepared, addressed, transmitted, routed, and received. The chapter begins by mapping the TCP/IP suite to the OSI model, then introduces common application protocols, port numbers, and secure protocol alternatives used for web access, file transfer, email, and remote administration.

The chapter also examines essential network services. DHCP and APIPA provide host configuration, while SNMP, Syslog, and NTP support network monitoring, event logging, and time synchronization. At the transport layer, TCP and UDP are compared, followed by a closer look at TCP connections, headers, flags, and the three-way handshake.

Finally, the chapter explains IP operation, including TTL, MTU, fragmentation, and protocol numbers, together with the supporting roles of ICMP and ARP. It also introduces GRE and IPsec, data encapsulation and decapsulation, and basic PowerShell commands used to inspect configurations, test connectivity, and troubleshoot TCP/IP communication.

15 sectionsPort explorerTCP handshakeDORA simulatorEncapsulation builder15-question exam check

TCP/IP and OSI Mapping

TCP/IP is the open protocol suite used by the Internet and most private networks. Its four layers combine the seven OSI layers into a practical implementation model.

TCP/IP layer	OSI equivalent	Primary responsibility
Process/Application	7, 6, 5	Application services, representation, and sessions
Host-to-Host	4 Transport	End-to-end delivery, reliability, and ports
Internet	3 Network	Logical addressing, routing, and packets
Network Access	2 and 1	Frames, MAC addresses, media, and signals

Mapping ruleHost-to-Host = Transport. Internet = Network. Process/Application combines OSI 5–7. Network Access combines OSI 1–2.

Protocol & Port Explorer

Select a protocol to connect its port, transport, purpose, and security clue.

Secure Replacement Flip Cards

Flip each older service to reveal the secure exam choice.

Telnet · TCP 23

Secure terminal?

SSH · TCP 22. Encrypts authentication and terminal traffic.

FTP · TCP 20/21

Secure file transfer?

SFTP · TCP 22. Transfers files through SSH; it is not FTP with TLS.

HTTP · TCP 80

Secure web?

HTTPS · TCP 443. Protects browser-server traffic with TLS.

LDAP · TCP 389

Secure directory?

LDAPS · TCP 636. Encrypts directory queries and credentials.

SMTP · TCP 25

Secure submission?

Submission · TCP 587. TLS protects submitted email in transit.

Exam trapSFTP is a separate SSH-based service on TCP 22—not “FTP with TLS.”

DHCP DORA & APIPA

DHCP uses UDP 67 on the server and UDP 68 on the client to deliver addressing, gateway, DNS, and domain information.

Client starts at 0.0.0.0 and broadcasts DHCPDISCOVER.

APIPA clueAn unexpected 169.254.x.x address means the Windows client could not reach DHCP or receive a valid lease.

Monitoring, Logging & Time

Syslog level	Severity	Meaning
0	Emergency	System unusable
1–3	Alert–Error	Immediate or serious condition
4–5	Warning–Notice	Potential problem or notable event
6–7	Information–Debug	Normal detail or diagnostics

Severity ruleLower syslog number means greater severity: 0 is Emergency; 7 is Debugging.

TCP and UDP Transport Protocols

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are transport-layer protocols that carry data between applications on network devices. Both protocols use port numbers to identify the sending and receiving applications, but they have different objectives and communication methods.

Connection-oriented

Transmission Control Protocol (TCP)

TCP establishes a logical connection before transmitting data. It uses sequencing, acknowledgments, error checking, flow control, and retransmission to ensure that data arrives completely and in the correct order.

Objective: To provide reliable, ordered, and error-checked delivery between applications.

Connectionless

User Datagram Protocol (UDP)

UDP sends datagrams without first establishing a connection. It does not provide acknowledgments, sequencing, flow control, or retransmission, resulting in lower overhead and faster delivery.

Objective: To provide fast and efficient delivery for applications that can tolerate some data loss or handle reliability themselves.

Choosing a protocol

Reliability versus speed

TCP is suitable for web browsing, email, file transfer, and remote administration, where complete delivery is important. UDP is suitable for DNS, DHCP, voice, video streaming, online gaming, monitoring, and broadcast traffic, where low delay is preferred.

Important Describing UDP as “unreliable” does not mean that it is a poor protocol. UDP deliberately omits delivery-recovery features to reduce delay and overhead. Reliability may be unnecessary for the application or may be managed at the application layer.

TCP Session Animator

A TCP connection starts by synchronizing sequence numbers. Step through SYN, SYN/ACK, and ACK.

No virtual circuit yet.

TCP Header and Control Flags

A TCP header contains the information needed to establish a connection, arrange segments in the correct order, acknowledge received data, control transmission, and detect errors. A standard TCP header is at least 20 bytes long and may be larger when optional fields are used.

TCP Segment Header Each row represents 32 bits

Source Port 16 bits

Destination Port 16 bits

Sequence Number 32 bits — identifies the position of data

Acknowledgment Number 32 bits — identifies the next expected byte

Data Offset Header size

Reserved Future use

Control Flags

URG ACK PSH RST SYN FIN

Window Size Flow control

Checksum 16 bits — detects errors

Urgent Pointer 16 bits — locates urgent data

Options and Padding Optional settings and alignment

Application Data Payload carried by the TCP segment

Exam focus TCP uses the sequence number, acknowledgment number, control flags, and window size to provide reliable, ordered, and flow-controlled delivery.

Principal TCP Control Flags

Flag	Meaning	Exam clue
SYN	Synchronizes sequence numbers between devices.	Starts a TCP connection.
ACK	Indicates that the acknowledgment field is valid.	Confirms received data.
FIN	Indicates that the sender has finished transmitting.	Gracefully closes a connection.
RST	Immediately resets an existing or attempted connection.	Aborts or rejects a connection.
PSH	Requests immediate delivery to the receiving application.	Process the data promptly.
URG	Indicates that the urgent pointer field is valid.	Marks urgent data.

Internet Protocol, TTL & MTU

IP provides logical addressing and routing on a best-effort basis. It does not guarantee delivery, order, or recovery.

Loop prevention

TTL

Each router decrements Time to Live. A packet is discarded when TTL reaches zero.

Packet size

MTU

Largest Layer 3 packet a link carries without fragmentation. Path-MTU problems may break large transfers while small packets work.

Next payload

Protocol field

Identifies ICMP 1, TCP 6, UDP 17, GRE 47, or another protocol carried inside IPv4.

Exam trapPorts identify applications inside TCP or UDP. The IPv4 Protocol field identifies the carried protocol. TCP is protocol 6—not “port 6.”

IP Protocol Number Explorer

ICMP & ARP

Control and errors

ICMP

Ping uses Echo Request/Reply. Tracert uses TTL expiry and Time Exceeded. A blocked ping alone does not prove a host is down.

IPv4 to local MAC

ARP

Checks cache, broadcasts a request on the local link, and learns the owner’s MAC address.

ARP decision lab

A client at 192.168.10.25/24 sends data to 198.51.100.20. Whose MAC address must it resolve?

Choose one answer.

Core ruleFor a remote destination, ARP resolves the local default gateway’s MAC. The IP destination remains the remote host.

GRE & IPSec

Flexible tunnel

GRE

Carries multiple Layer 3 protocols and multicast, but provides no encryption, authentication, or confidentiality.

Authenticate

AH

Provides origin authentication and integrity, but no encryption; address changes such as NAT are problematic.

Encrypt

ESP

Provides confidentiality plus optional integrity, authentication, and anti-replay protection.

Negotiate

IKE

Negotiates peers, security associations, algorithms, parameters, and keys.

Memory hookAH authenticates. ESP encrypts and can authenticate. IKE negotiates. GRE gains security when protected by IPSec.

Encapsulation Builder

Build the sending sequence from application data to physical bits.

Start with application data.

OrderData → Segment/Datagram → Packet → Frame → Bits. Receiving devices reverse the process.

PowerShell 7 Practice

Get-NetTCPConnectionTCP listeners and connections

Get-NetUDPEndpointLocal UDP endpoints

Test-NetConnection example.com -Port 443DNS and TCP port test

Get-NetNeighborARP / neighbor cache

Get-NetRouteLocal routing table

ping example.comICMP Echo test

tracert example.comTTL-based routed path

ipconfig /allDHCP, DNS, and addressing

Exam Check

Fifteen questions drawn directly from the Chapter 6 manual.

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