Network Segmentation Review

Network Segmentation Review

Verify that intended segmentation boundaries actually hold.

1. Understand the Intended Design

• Obtain the network diagram: zones (DMZ, internal, production, corporate, OT), trust levels, intended flows
• Map documented allowed flows vs "everything else should be denied"

2. Verify Layer 2/3 Boundaries

• VLAN hopping: double-tagging, DTP negotiation on user ports, native VLAN mismatches on trunks
• Inter-VLAN routing: which SVIs exist, ACLs between them
• Private VLANs / port isolation where host-to-host traffic is unnecessary

3. Firewall and Router Policy

• Rulebase review: any-any rules, overly broad source/destination (any→internal), rules by IP instead of objects
• Shadowed and unused rules; logging disabled on deny rules
• Management access: SSH/admin planes reachable from user segments?

4. Cloud Boundaries

• Security groups: per-role groups, no 0.0.0.0/0 ingress on non-web ports
• NACLs vs SGs layering; peering/transit gateway route tables leaking broader CIDRs
• Egress control: can compromised workload reach arbitrary internet (data exfil path)?

5. Active Validation (authorized testing)

• From representative hosts in each zone, attempt connections across boundaries: allowed (should work), denied (should fail and log)
• Test both directions and return traffic for stateful devices
• Use nmap -PN -sT -p <port> style targeted probes; record device logging the block

Output

A verified flow matrix (source zone × destination zone × port × expected × observed), gaps with device-level evidence, and remediation per boundary.