Glossary of dedicated server and colocation terms

Hardware and the rack

U (rack unit): 44.45 mm of vertical space in a rack. A typical server is 1U or 2U; a full cabinet is usually 42U or 47U.
Rail kit: The sliding mounts a server sits on. Without one, a server is sitting on a shelf or on the machine below it — neither is acceptable in a shared rack.
PDU: Power Distribution Unit. The vertical strip of sockets in the rack. A switched PDU allows a socket to be power-cycled remotely.
BMC: Baseboard Management Controller. A small computer on the motherboard that runs whether or not the server is on, and gives console, power control and sensor readings. See IPMI and the BMC.
IPMI: The protocol used to talk to a BMC. In everyday speech, people say IPMI when they mean the BMC's web interface.
KVM: Keyboard, Video, Mouse — remote console as if you were standing in front of the machine. Confusingly, also the name of the Linux hypervisor. Context decides.
ECC: Error-Correcting Code memory. Detects and corrects single-bit errors instead of corrupting your data silently. Standard in every machine we build.
Hot-swap: A component that can be replaced while the machine runs — usually drives and power supplies.

SAS: Serial Attached SCSI. The interface used in servers: dual-ported, better error reporting, built for continuous duty. What we fit by default.
SATA: The consumer-and-light-server interface. Cheaper, single-ported, shorter duty expectations. See SAS, SATA and NVMe.
NVMe: Flash attached straight to PCI Express, bypassing the old disk command stack. The fastest option by a wide margin for small random operations.
RAID: Combining several drives for redundancy, speed, or both. Not a backup. See RAID and disks.
IOPS: Input/Output Operations Per Second. The number that matters for databases and busy mail servers — far more than raw megabytes per second.

BGP: Border Gateway Protocol, how networks on the internet tell each other which addresses they carry. See running BGP with us.
ASN: Autonomous System Number. The identifier for a network that speaks BGP.
RIR / LIR: Regional Internet Registry (RIPE NCC, in our region) and Local Internet Registry — the bodies that allocate address space.
IXP: Internet Exchange Point, where networks interconnect directly. AMS-IX in Amsterdam is one of the largest in the world.
Transit and peering: Transit is paying a carrier to reach the whole internet. Peering is exchanging traffic directly with another network, usually settlement-free. A good network has plenty of both.
PTR / reverse DNS: The record mapping an address back to a name. Mail will not work properly without it. See reverse DNS.
LACP: The standard for bundling several physical links into one logical link. See bonding and LACP.
MTU: The largest packet a link will carry, normally 1500 bytes. See MTU and fragmentation.
CIDR / prefix: The /24 or /48 notation for a block of addresses. The smaller the number, the larger the block.
Unmetered: A port sold by its speed with no transfer counter. See what unmetered actually means.

Bare metal: A physical server with nothing between you and the hardware. The same thing as a dedicated server; the newer term exists to distinguish it from cloud instances.
Colocation: You own the hardware, we provide the rack, power, cooling and network.
Unmanaged: We keep the hardware and network running; the operating system and everything above it is yours. See what unmanaged means.
Remote hands: Our engineers doing physical work on your behalf — reseating a drive, swapping a cable, pressing a button.
SLA: Service Level Agreement. The written commitment on availability and response, and what happens when it is missed. Ours is published in full.
RMA: Return Merchandise Authorisation — the process for sending faulty hardware back to a manufacturer.

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