Learn how data travels across long distances using WAN technologies, and discover the logic routers use to forward packets through complex networks.
Why it matters
Understanding WANs and IP routing is crucial because most networks are interconnected. When you access a website, your data likely traverses multiple WAN links and routers. This chapter explains how routers make forwarding decisions and how different WAN technologies enable long-distance communication.
What You’ll Learn
🏢 WAN Technologies
Leased lines, HDLC, and Ethernet as WAN alternatives
🧭 IP Routing Logic
How routers forward packets step-by-step across networks
🌐 DNS & ARP
Name resolution and address mapping protocols
📊 Routing Protocols
How routers learn and share network paths
Assessment
“Do I Know This Already?” Quiz
Test your existing knowledge before diving into the chapter.
1. Which device typically connects a LAN to a WAN?
Routers connect different networks, including LAN-to-WAN connections.
2. What protocol resolves domain names to IP addresses?
DNS (Domain Name System) translates human-readable names to IP addresses.
3. What does ARP resolve?
ARP (Address Resolution Protocol) maps IP addresses to MAC addresses on the local network.
Foundation Topics
What is a Wide-Area Network?
A WAN connects networks across geographic distances, typically using telecommunications provider infrastructure.
LAN vs WAN Perspective
PC1
192.168.1.10
R1
Gateway
☁️ WAN Telco Network
R2
Remote
PC2
10.1.1.10
LAN 1
LAN 2
Key Point: WANs are typically owned and operated by telecommunications companies (telcos). Organizations lease WAN connectivity rather than building their own long-distance infrastructure.
WAN Characteristics
📏 Geographic Scope
Spans cities, countries, or continents—much larger than LANs
🏢 Telco Ownership
Infrastructure owned by telecommunications providers
💰 Leased Connections
Organizations pay for bandwidth and connectivity
Wide-Area Networks
Leased-Line WANs
A dedicated point-to-point connection between two sites, providing consistent bandwidth and privacy.
Leased Line Physical Components
🏢
Site A
CSU/DSU
📡
Router
Serial Interface
☁️
Telco Cloud
WAN Link
📡
Router
Serial Interface
Physical path through telco infrastructure
Physical Details
🔌 Serial Interfaces
Routers use serial ports to connect to WAN links, different from Ethernet LAN ports
📞 CSU/DSU
Channel Service Unit / Data Service Unit—converts router signals for telco lines
⏱️ Clocking
Synchronous communication requires timing signals to coordinate bit transmission
Key Advantage
Leased lines provide guaranteed bandwidth and privacy. Unlike shared internet connections, the entire capacity is dedicated to your organization.
Data-Link Layer
HDLC: WAN Data-Link Protocol
High-Level Data Link Control—Cisco’s default protocol for leased lines, providing framing and error detection.
Default on Cisco serial interfaces—works out of the box
Synchronous operation—uses clocking from CSU/DSU
Point-to-point only—designed for two-device connections
Error detection—CRC in FCS field catches transmission errors
No authentication—unlike PPP, HDLC has no built-in security
Comparison: HDLC is simpler and faster than PPP but lacks authentication and multi-protocol support. For basic Cisco-to-Cisco leased lines, HDLC is the default choice.
Modern WANs
Ethernet as a WAN Technology
Modern alternatives to leased lines use Ethernet standards over telco infrastructure, providing higher speeds and simpler management.
Ethernet over MPLS (EoMPLS)
🏢Site A
Ethernet
Fiber
☁️MPLS Cloud
Fiber
🏢Site B
Ethernet
Routers see a Layer 2 connection; MPLS cloud is transparent
Ethernet WAN Benefits
⚡ Higher Speeds
10 Mbps to 100 Gbps—much faster than traditional leased lines
🔧 Familiar Interface
Uses standard Ethernet ports—no special serial cables needed
☁️ E-Line Service
Ethernet Line (E-Line) provides point-to-point emulation
IP Routing
Network Layer Routing Logic
Routers forward packets hop-by-hop, making independent decisions based on destination IP addresses and routing tables.
Four-Step Routing Process
1
Host Sends to Default Gateway
When the destination is on a different network, the host sends the packet to its configured default router (gateway).
2
Router Consults Routing Table
Each router looks at the destination IP, finds the best matching route, and determines the next-hop router.
3
Encapsulation for Next Link
The router strips the old data-link header and adds a new one appropriate for the outgoing interface.
4
Final Delivery
The last router (connected to the destination network) delivers the packet directly to the destination host.
Encapsulation at Each Hop
📦 Layer 3: IP Packet (Destination IP: 10.1.1.10)
🔗 Layer 2: Data-Link Frame (MAC: R2’s MAC)
📡 Layer 1: Physical Signals on Wire
Layer 3 stays constant; Layer 2 changes at each hop
Network Layer
How IP Addressing Helps Routing
IP addresses are grouped into networks and subnets, allowing routers to summarize routes and make efficient forwarding decisions.
IP Network Grouping
🌐
Class A
1.0.0.0 – 126.0.0.0
/8 networks
🌐
Class B
128.0.0.0 – 191.255.0.0
/16 networks
🌐
Class C
192.0.0.0 – 223.255.255.0
/24 networks
IP Header Fields for Routing
Field
Purpose
Size
Destination IP
Identifies intended recipient; used for routing decisions
32 bits
Source IP
Identifies sender; used for return traffic
32 bits
TTL
Time To Live—decremented at each hop; prevents loops
8 bits
Protocol
Identifies payload type (TCP=6, UDP=17, ICMP=1)
8 bits
Key Concept
Routers don’t need to know about every individual host—just the networks. This allows routing tables to stay small even as networks grow large.
Dynamic Routing
How Routing Protocols Learn Routes
Routing protocols automate route learning, allowing routers to adapt to network changes without manual configuration.
Three-Step Process: Learning Routes
📊Learn
→
📢Advertise
→
🧭Choose
Routing Protocol Process
1️⃣ Learn About Subnets
Discover directly connected networks and listen to neighbor advertisements
2️⃣ Advertise Routes
Tell neighboring routers about reachable networks and their metrics
3️⃣ Choose Best Routes
Compare multiple paths and select the best based on metrics (hop count, bandwidth, delay)
Common Routing Protocols: OSPF (Open Shortest Path First), EIGRP (Enhanced Interior Gateway Routing Protocol), and BGP (Border Gateway Protocol) for internet routing.
Network Layer Features
DNS: Domain Name System
DNS translates human-readable names (like www.example.com) into IP addresses that routers use to forward packets.
DNS Name Resolution Process
💻
Host
“What’s www.cisco.com?”
Query
📚
DNS Server
Checks records
Response
IP
72.163.4.161
Now host can connect!
Why DNS Matters
Humans remember names, not numbers—easier to recall “google.com” than “142.250.80.46”
Allows IP changes—update DNS record without users noticing
Load distribution—one name can resolve to multiple IPs
Hierarchical structure—distributed across many servers worldwide
Network Layer Features
ARP: Address Resolution Protocol
ARP resolves IP addresses to MAC addresses, enabling Layer 3 packets to be encapsulated in Layer 2 frames on local networks.
ARP Request and Response
💻
ARP Request (Broadcast)
“Who has IP 192.168.1.1? Tell me (192.168.1.10)!”
📡
📡
ARP Reply (Unicast)
“I have 192.168.1.1. My MAC is 00:1A:2B:3C:4D:5E”
📡
How ARP Works
1️⃣ Check Cache
Host first checks its ARP cache for an existing mapping
2️⃣ Broadcast Request
If not found, send ARP request to FF:FF:FF:FF:FF:FF
3️⃣ Unicast Reply
Target host responds with its MAC address
4️⃣ Cache Entry
Mapping stored in ARP cache for future use
Network Layer Features
Ping and ICMP
ICMP (Internet Control Message Protocol) provides error reporting and diagnostic capabilities, with ping being the most common tool.
Ping Operation (ICMP Echo)
💻
Host A
ICMP Echo Request →
← ICMP Echo Reply
🖥️
Host B
Round-trip time measured to test connectivity
Common ICMP Types
Type
Name
Purpose
0
Echo Reply
Response to ping request
8
Echo Request
Ping request
3
Destination Unreachable
Host or network not reachable
11
Time Exceeded
TTL reached zero (traceroute uses this)
Review
Key Terms You Should Know
Click each term to reveal its definition.
Leased Line
A dedicated point-to-point WAN connection rented from a telecommunications provider, providing guaranteed bandwidth between two sites.
WAN (Wide-Area Network)
A network that spans a large geographic area, connecting multiple LANs, typically using telco infrastructure.
Telco
Telecommunications company—provides WAN connectivity and infrastructure.
Serial Interface
A router interface designed for WAN connections, transmitting one bit at a time.
HDLC
High-Level Data Link Control—default Cisco protocol for leased lines, providing framing and error detection.
Ethernet over MPLS (EoMPLS)
A technology that transports Ethernet frames over MPLS networks, allowing Ethernet as a WAN service.
E-Line (Ethernet Line Service)
A point-to-point Ethernet WAN service that emulates a leased line.
Default Router/Gateway
The router on a local network that hosts use to reach destinations on other networks.
Routing Table
A data structure in a router containing routes to destination networks and their next-hop information.
IP Network/Subnet
A group of IP addresses that share a common network portion, allowing routers to summarize routes.
IP Packet
A Layer 3 PDU containing source/destination IP addresses, TTL, protocol, and payload.
Routing Protocol
A protocol that allows routers to exchange route information and dynamically learn network topology.
Dotted-Decimal Notation (DDN)
The format for writing IPv4 addresses: four decimal numbers separated by dots (e.g., 192.168.1.1).
DNS (Domain Name System)
A distributed system that translates human-readable domain names to IP addresses.
ARP (Address Resolution Protocol)
Resolves IP addresses to MAC addresses on the local network.
Ping
A tool using ICMP echo requests/replies to test network connectivity and measure latency.
Assessment
Chapter 3 Quiz
Test your knowledge of WANs and IP routing.
1. What device connects a LAN to a WAN?
Routers connect different networks, including LAN-to-WAN boundaries.
2. Which protocol is Cisco’s default for leased lines?
HDLC (High-Level Data Link Control) is the default encapsulation on Cisco serial interfaces.
3. What does ARP resolve?
ARP (Address Resolution Protocol) maps IP addresses to MAC addresses for local delivery.
4. What protocol translates domain names to IP addresses?
DNS (Domain Name System) resolves hostnames like www.example.com to IP addresses.
5. What ICMP type does ping use for requests?
ICMP Echo Request is Type 8; Echo Reply is Type 0.
6. What field in the IP header prevents infinite loops?
TTL (Time To Live) is decremented at each hop; when it reaches zero, the packet is dropped.
7. Which is a characteristic of leased lines?
Leased lines are dedicated point-to-point connections between two sites.
8. What does a routing protocol do?
Routing protocols allow routers to dynamically learn network topology and share routes.
9. What layer does IP operate at?
IP is a Layer 3 (Network Layer) protocol.
10. What is EoMPLS?
EoMPLS (Ethernet over MPLS) transports Ethernet frames across MPLS networks, enabling Ethernet WAN services.