Segments

Network Segments

  • A segment is a single collision domain where devices share the same physical medium and compete for access
  • Key principle: Only one device can transmit at a time within a segment to avoid collisions
  • Segments are bounded by Layer 2 devices (switches/bridges) or Layer 3 devices (routers)

Physical vs Logical Segmentation

  • Physical segments: Separated by actual hardware boundaries (switch ports, router interfaces)
  • Logical segments: Separated by VLANs, subnets, or other software-defined boundaries
  • Modern networks primarily use logical segmentation for flexibility and cost efficiency

Collision Domains vs Broadcast Domains

Aspect Collision Domain Broadcast Domain
Boundary Switch ports, router interfaces Router interfaces only
Traffic Type Ethernet collisions Broadcast/multicast frames
Devices per Domain Ideally 1 (full-duplex switching) Multiple subnets possible
Performance Impact Collisions reduce throughput Broadcasts consume bandwidth

Segmentation Benefits

  • Collision reduction: Each switch port = separate collision domain
  • Bandwidth optimization: Full bandwidth available per segment (no sharing)
  • Security isolation: Separate segments can have different access policies
  • Broadcast control: Routers block broadcasts between segments
  • Fault containment: Problems in one segment don’t affect others

Implementation Examples

  • Hub networks: Single segment = entire hub (legacy, avoid in production)
  • Switched networks: Each port = separate segment (modern standard)
  • VLAN segmentation: Logical separation within same physical switch
  • Routed segmentation: Different IP subnets on separate router interfaces

Vocabulary

  • Collision Domain: Network segment where data collisions can occur
  • Broadcast Domain: Network segment where broadcast frames are propagated
  • Microsegmentation: Creating individual collision domains for each device
  • Trunk Port: Carries multiple VLAN segments between switches

Notes

  • Modern Ethernet is full-duplex: Eliminates collisions in switched environments (separate transmit/receive paths)
  • CSMA/CD is disabled on full-duplex links since collisions are impossible
  • VLAN design rule: Keep broadcast domains appropriately sized (typically 200-500 hosts max)
  • Wireless networks still operate as shared segments within each frequency channel
  • Troubleshooting tip: Use show interfaces to check for collisions on half-duplex links
  • Security consideration: Segment sensitive systems into separate VLANs or subnets with ACL controls