Articles
Access and trunk ports: one VLAN on a port, or many over one link
What an access port and a trunk port each carry, how untagged and IEEE 802.1Q-tagged traffic differ on a trunk, and how to configure and verify both on Cisco IOS XE.
Networking
Protocols, routing, VLANs and network infrastructure.
Articles
ARP and Neighbor Discovery: resolving the next hop, not the destination
Why IPv4 ARP and IPv6 Neighbor Discovery map a layer-3 address to a layer-2 address on the local link, what each protocol actually does, and where their trust model breaks.
Articles
What ping, traceroute, ipconfig and dig can and cannot prove
What each basic network tool really answers, how to read its result, and the conclusions it does not support.
Articles
DHCP scope, lease, reservation and relay
How a DHCP server is scoped to a subnet, how leases and reservations decide which address a client gets, and how a relay agent lets one server serve other segments.
Articles
DNS and DHCP: names, addresses and automatic configuration
What DNS and DHCP each solve, how resolution and address assignment actually work, and why both are unauthenticated by default.
Articles
DNS records and resolution: which record answers a query, and where it comes from
How the common DNS record types answer different questions, how a resolver walks the delegations to the authoritative server that owns a name, and how a Cisco IOS-XE device resolves and serves names.
Articles
Ethernet frames, MAC addresses and how switches learn
What an Ethernet frame carries, what a MAC address identifies, and how a switch builds the table it forwards from.
Articles
ICMP, TCP, UDP, ports and sockets: how IP reaches the right process
What ICMP does inside IP, how TCP and UDP differ, and why a port identifies a process rather than a trusted service.
Articles
Inter-VLAN routing: how traffic crosses between VLANs
Why two VLANs cannot reach each other at Layer 2, and how one Layer-3 interface per VLAN — a router subinterface or a switch SVI — routes traffic between them.
Articles
IPv4 and IPv6: prefixes, subnets and the default gateway
How an address splits into a prefix and a host part, why a prefix boundary can fall inside an octet, and what the default gateway actually does in IPv4 and IPv6.
Articles
Link aggregation and LACP: making several links look like one
How link aggregation bundles parallel links into one logical interface, what LACP negotiates before the bundle is used, and why one flow still moves at one link speed.
Articles
The native VLAN: what it is and why the design choice matters
What the native VLAN means on an 802.1Q trunk, how untagged traffic is classified, and why the choice affects spanning tree and segmentation.
Articles
Switch, router, firewall and access point: what each device decides on
Four devices, four different decisions: the information each one uses, the scope it applies in, and why "which layer it is" is the wrong question.
Articles
NTP and time synchronisation: keeping network clocks aligned
How NTP keeps network devices on the same clock - the server hierarchy, the round-trip measurement, and why unauthenticated time is a risk.
Articles
OSI and TCP/IP: how a packet actually travels
Why the layered models exist, what each layer adds to a packet, and where OSI and TCP/IP differ.
Articles
PoE and physical links: power, speed and duplex at the port
How a switch port powers a device over the cable (PoE) and how both ends agree on speed and duplex — and the Cisco IOS-XE commands used to inspect and control them.
Articles
The routing table, the next hop and the default gateway
What a routing table entry contains, how a device picks the longest matching prefix and its next hop, and what the default gateway and the default route actually are.
Articles
Static routes and default routes: telling a router where to forward
How a static route and a default route are written on Cisco IOS-XE, how the router chooses between routes, and when a manually configured route fails silently.
Articles
STP and loop prevention: keeping a Layer 2 network loop-free
Why a Layer 2 loop becomes a broadcast storm, how STP elects a root and blocks the ports that would close a loop, and how to configure and verify it on Cisco IOS-XE.
Articles
Switching and MAC learning in operation: reading and controlling the MAC address table
How a Cisco IOS-XE switch stores what it has learned, how to read, populate, age and clear the MAC address table, and what its entries actually tell you about a Layer 2 network.
Articles
Unicast, broadcast and multicast: one host, one group, or a whole segment
How unicast, broadcast and multicast address traffic, why IPv6 dropped broadcast, and why the size of a broadcast domain is an operational problem.
Articles
VLANs and 802.1Q tagging: how one link carries many networks
How IEEE 802.1Q lets a single link carry many VLANs — the 4-byte tag, access and trunk ports, and the native VLAN.