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Subnetting is one of those skills that seems intimidating until it clicks, and then it feels obvious. This post builds from first principles — binary representation, masks, and block sizes — through variable-length subnet masking (VLSM) and summarization. IP Address Structure An IPv4 address is 32 bits divided into two logical parts: network and host. 192.168.10.25 = 11000000.10101000.00001010.00011001 The subnet mask defines the split. A 255.255.255.0 mask (or /24 in CIDR) means the first 24 bits identify the network, the last 8 identify the host. ...

OSPF link-state advertisements are the currency of the protocol. Understanding which LSA type carries what information — and who generates it — is essential for troubleshooting convergence issues. Quick Reference Type Name Originated By Scope 1 Router LSA Every router Single area 2 Network LSA DR on broadcast segment Single area 3 Summary LSA ABR Area → backbone 4 ASBR Summary ABR Area → backbone 5 AS External ASBR Entire OSPF domain 7 NSSA External ASBR in NSSA Single NSSA area Type 1 — Router LSA Every OSPF router originates one Type 1 LSA per area it belongs to. It describes: ...