The Elematron blog

Network engineering, explained and fixed.

Short, correct explainers on the problems engineers actually hit — each paired with a free tool and a daily drill so you can practise what you just read.

Subnetting4 min read

VLSM subnetting without the headache: a repeatable method

Learn a systematic VLSM subnetting method that eliminates guesswork. Step-by-step process for variable-length subnet masks with real examples.

DSCP marking explained: EF, AF classes and QoS

Learn how DSCP marking classifies traffic for QoS. Understand EF, AF classes, CS values, and practical marking strategies for enterprise networks.

Routing4 min read

Administrative distance: how a router picks between OSPF, EIGRP and static

Learn how administrative distance works and why routers prefer one routing protocol over another. Complete AD reference table included.

Security4 min read

GRE, ESP, and OSPF: Why they have no port numbers

Learn why GRE, ESP, and OSPF use protocol numbers instead of ports, and how to write ACLs for these Layer 3 protocols.

Troubleshooting4 min read

MTU, MSS, and why your VPN tunnels drop large packets

Learn why VPN tunnels fragment packets, how MTU and MSS interact, and how to diagnose and fix packet loss on encrypted connections.

Addressing4 min read

169.254.x.x address on your interface: what APIPA really means

Learn what APIPA (169.254.x.x) means, when it appears on your network interface, and how to troubleshoot it.

Routing4 min read

OSPF cost reference bandwidth: why 100 Mbps links get cost 1

By default OSPF gives every link above 100 Mbps a cost of 1, so it cannot tell 1G from 100G apart. Why it happens and how to fix it with reference bandwidth.

Reference3 min read

The ports every network engineer should know cold

You should never have to look up 22, 53, 443 or 179 mid-incident. A tour of the well-known ports that matter most for firewalls, ACLs and troubleshooting.

Addressing3 min read

Reading a MAC address: OUI, unicast and the local bit

A MAC address is not just 12 random hex digits. The first half identifies the vendor, and two bits in the first byte tell you unicast vs multicast and universal vs local.

IPv6 address types (and how to compress them)

Global unicast, link-local, unique local, multicast - IPv6 scopes trip up engineers used to IPv4. A quick map of the ranges, plus the two rules for compressing any address.

Least privilege3 min read

Do these two subnets overlap? A 30-second check

Overlapping subnets quietly break routing, ACLs and firewall rules. The fast way to tell whether two CIDR blocks overlap, are disjoint, or one contains the other.

Routing3 min read

Route summarization: turn many routes into a few

Advertising four consecutive /24s as four routes is four times the work for your neighbours. Summarize them into one /22. How aggregation works and the trap to avoid.

Subnetting4 min read

VLSM: subnet one network for wildly different host counts

Fixed-size subnets waste addresses when your segments range from 2 hosts to 500. Variable Length Subnet Masking sizes each subnet to its need. The method, step by step.

Troubleshooting4 min read

Small requests work, big ones hang: the IPsec MTU trap

After bringing up a site-to-site VPN, some HTTPS pages load half-way then stall while pings and small requests are fine. That is an MTU/MSS problem. Here is the clamp.

DSCP markings decoded: EF, AF and CS without the guesswork

EF is 46, CS6 is 48, AF41 is 34 — but what do they mean and when do you use each? A plain-English guide to DSCP QoS markings for network engineers.

Routing4 min read

Administrative distance: how your router picks between two routes

When the same prefix is learned from OSPF and BGP, which one wins? Administrative distance decides — before metric ever enters the picture. The default table and how to use it for failover.

Least privilege3 min read

Subnet mask vs wildcard mask: stop mixing them up

A subnet mask and a Cisco wildcard mask are bitwise inverses. Confusing them writes ACLs and OSPF network statements that match the wrong hosts. The simple rule, plus the /12 trap.

Troubleshooting4 min read

OSPF stuck in EXSTART/EXCHANGE? It is almost always MTU

When an OSPF adjacency hangs in EXSTART or EXCHANGE and never reaches FULL, the cause is nearly always an interface MTU mismatch. Why it happens, how to confirm it, and how to fix it.