NAT in Four Flavours — Static, Dynamic, PAT
NAT (Network Address Translation) translates IP addresses (and sometimes ports) between private and public networks. Facts Term Explanation Inside local The private IP address (before NAT) Inside global The public IP address (after NAT) Outside global The actual IP of the external destination Outside local How the external IP is seen internally NAT table Mapping […]
QoS — EF, LLQ & the Trust Boundary
QoS ensures that delay-sensitive traffic (voice and video) is prioritized correctly under network load. Facts Term Explanation Latency Delay from sender to receiver Jitter Variation in latency (very critical for voice) Packet loss Lost packets (very critical for voice/video) Bandwidth Available capacity Congestion Traffic exceeds link capacity Classification Identifying traffic Marking Marking traffic (DSCP/CoS) Queuing […]
ICMP — The Error Messages That Explain the Network
ICMP is used for error handling and diagnostics in IP networks by sending control and error messages. Facts (CCNA) ICMP is part of the IP layer (Layer 3) – not TCP/UDP Used for error reporting and diagnostics, not data transport Known tools: ping and traceroute ICMP messages are sent in response to IP packets that […]
DNS — Hierarchy, Records & TTL
DNS is a critical network service that translates domain names (e.g. www.cisco.com) into IP addresses. Without DNS users would have to remember IP addresses. Facts Point Description Function Name lookup: name → IP (and reverse) Transport UDP/53 standard; TCP/53 for zone transfers and large answers Architecture Hierarchical, distributed database Security Vulnerable to spoofing/poisoning → DNSSEC […]
NTP — Strata, Peers & Authentication
NTP makes sure every network device has the same precise time so that logs, security and troubleshooting make sense. Facts Point Detail Purpose Synchronize time on network devices Transport UDP port 123 Model Hierarchical stratum model Accuracy Milliseconds (LAN), typically <100 ms over WAN/internet Alternative PTP (very high precision, hardware dependent) Used for Logs, syslog, […]
SLAAC vs DHCPv6
SLAAC: the IPv6 client builds its own address based on Router Advertisements (RA). DHCPv6: a server assigns IPv6 addresses and/or other parameters – stateless (client still uses SLAAC, DHCPv6 only supplies DNS etc.) or stateful (the server assigns the address itself and tracks clients). Comparison Point SLAAC DHCPv6 stateless DHCPv6 stateful Assigns IPv6 address ✅ […]
DHCP — DORA, Relay & Options
DHCP is a network service that automatically assigns IP addresses and other IP parameters to clients using a central server, with a relay agent across subnets if needed. Facts Topic Fact Transport UDP Server port UDP 67 Client port UDP 68 IPv4 process DORA (Discover, Offer, Request, Acknowledge) Broadcast The client always starts with broadcast […]
ARP — Requests, Caches & Gratuitous
ARP translates IPv4 addresses into MAC addresses so Layer 3 can communicate correctly over Layer 2 on the local network. Facts (key topics) ARP is used only inside the same broadcast domain (LAN) ARP is stateless and unauthenticated (security risk) ARP cache reduces broadcast traffic and latency Gratuitous ARP is used for IP conflict detection […]
Summarization vs Aggregation (Null0)
Route summarization/aggregation combines several more specific routes into one summary route: 10.1.1.0/24 10.1.2.0/24 → 10.1.0.0/16 Why use it? Smaller routing tables · fewer routing updates · faster convergence Hides internal network details · better scalability How it works The router advertises one aggregated prefix Longest Prefix Match always prefers the more specific route Traffic to […]
Route Redistribution — And Its Loop Traps
Route redistribution shares routes between different routing protocols – but introduces complexity and potential routing loops. It happens on a router running multiple protocols, which acts as the redistribution point, translating routes from one protocol’s metric to another’s. When is it used? Networks with multiple routing protocols Migration between protocols Joining administrative domains Facts (CCNA) […]