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Routing Infrastructure Considerations

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Summary

PDF p.117

Layer 3 forwarding, or routing, uses logical addressing to identify networks and subnets, each being a separate broadcast domain. Nodes are identified by IP addresses, and links by routes. Virtual LANs (VLANs) help map logical IP topology to physical hardware switches, creating separate Layer 2 domains that can be mapped to Layer 3 IP subnets.

In plain words

Supplementary — not from your PDF

Routing (layer 3) uses IP addressing to split networks into subnets, and each subnet is its own broadcast domain. VLANs let one physical switch carry several separate layer 2 networks. Each VLAN maps to an IP subnet, and traffic between VLANs must go through a router, where access rules can be applied.

Detailed explanation

PDF p.117
  • Layer 3 Forwarding (Routing)
    • Logical Addressing: Identifies networks and subnets.
    • Segmentation: Networks within networks (subnets).
    • Broadcast Domains: Each subnet is a separate broadcast domain.
    • Identification: Nodes by IP addresses, links by routes.
  • Internet Protocol (IP)
    • IPv4 Addressing
      • 32-bit Address: Written in dotted decimal notation.
      • Network Prefix/Subnet Mask: Divides address into network ID and host ID.
      • Example: 10.1.1.0/24 (network ID: 10.1.1.x, host ID: x).
    • IPv6 Addressing
      • 128-bit Address: Written in hex notation.
      • Hierarchy: First 64-bits for network information, last 64-bits for host's interface ID.
      • Example: 2001:db8::abc:0:def0:1234.
  • Address Resolution
    • IPv4: Uses Address Resolution Protocol (ARP) to map IP to MAC address.
    • IPv6: Uses Neighbor Discovery (ND) protocol for the same purpose.
  • Virtual LANs (VLANs)
    • Logical IP Topology: Mapped to physical hardware switches.
    • VLAN IDs: Value from 2 to 4,094.
    • Port Assignment: Any switch port can be assigned to a specific VLAN.
    • Layer 2 Domains: Each VLAN is a separate Layer 2 domain.
    • Layer 3 Mapping: VLANs mapped to IP subnets at Layer 3.
  • Example
    • Access Block: Uses VLANs to segment workstation hosts (VLAN32) from VoIP handsets (VLAN40).
    • Subnets: 10.1.32.0/24 and 10.1.40.0/24.
    • Router Usage: Required for communication between VLANs.
    • Access Control: Rules on the router can prevent risky communication.
  • Extended VLAN Topology
    • Multiple Switches: VLAN topology can extend across multiple switches.
    • Office Expansion: Same VLAN IDs and subnets configured for different floors.

Important terms

taken from the text above
Logical Addressing
Identifies networks and subnets.
Segmentation
Networks within networks (subnets).
Broadcast Domains
Each subnet is a separate broadcast domain.
Identification
Nodes by IP addresses, links by routes.
32-bit Address
Written in dotted decimal notation.
Network Prefix/Subnet Mask
Divides address into network ID and host ID.
128-bit Address
Written in hex notation.
Hierarchy
First 64-bits for network information, last 64-bits for host's interface ID.
IPv4
Uses Address Resolution Protocol (ARP) to map IP to MAC address.
IPv6
Uses Neighbor Discovery (ND) protocol for the same purpose.
Logical IP Topology
Mapped to physical hardware switches.
VLAN IDs
Value from 2 to 4,094.
Port Assignment
Any switch port can be assigned to a specific VLAN.
Layer 2 Domains
Each VLAN is a separate Layer 2 domain.
Layer 3 Mapping
VLANs mapped to IP subnets at Layer 3.
Access Block
Uses VLANs to segment workstation hosts (VLAN32) from VoIP handsets (VLAN40).
Subnets
10.1.32.0/24 and 10.1.40.0/24.
Router Usage
Required for communication between VLANs.
Access Control
Rules on the router can prevent risky communication.
Multiple Switches
VLAN topology can extend across multiple switches.
Office Expansion
Same VLAN IDs and subnets configured for different floors.
VLANs Virtual LANs IP Internet Protocol ARP Address Resolution Protocol ND Neighbor Discovery

Examples & real-world scenarios

Supplementary — not from your PDF
  • 10.1.1.0/24: the /24 prefix means the first 24 bits are the network ID.
  • Workstations on VLAN32 and VoIP phones on VLAN40, each with its own subnet.
  • ARP (IPv4) or Neighbor Discovery (IPv6) mapping an IP address to a MAC address.

Scenario

IP phones and PCs share one floor. Putting phones on their own VLAN and subnet means a compromised PC can't freely reach the phones. A router ACL decides what can cross between them.

Common mistakes

Supplementary — not from your PDF
  • Thinking VLANs alone block traffic. They separate layer 2, but a router between them decides what's allowed.
  • Mixing up IPv4 (32-bit, dotted decimal) and IPv6 (128-bit, hexadecimal).

Practical skills

Supplementary — not from your PDF
  • Read a CIDR prefix and explain what it says about network and host IDs.
  • Explain how VLANs map to subnets.

What I should remember

Key Points PDF p.117
  • Layer 3 Forwarding
    • Logical Addressing: Networks and subnets.
    • Broadcast Domains: Separate for each subnet.
  • Internet Protocol
    • IPv4: 32-bit, dotted decimal, network prefix/subnet mask.
    • IPv6: 128-bit, hex notation, hierarchical addressing.
  • Address Resolution
    • IPv4: ARP.
    • IPv6: ND protocol.
  • Virtual LANs (VLANs)
    • Logical to Physical Mapping: VLAN IDs, port assignment.
    • Layer 2 Domains: Separate for each VLAN.
    • Layer 3 Mapping: VLANs to IP subnets.
  • Example
    • Access Block: VLAN32 (workstations), VLAN40 (VoIP).
    • Subnets: 10.1.32.0/24, 10.1.40.0/24.
    • Router: Required for inter-VLAN communication.
    • Access Control: Router rules for security.
  • Extended VLAN Topology
    • Multiple Switches: Same VLAN IDs and subnets across floors.