[#rackunit]: “rack unit”

The term “rack unit” may refer to something which is the height of “1U” in height.

There is some variation. Wikipedia's article for “19-inch rack”, sub-section called “Rack types” has stated, “A standard 19-inch server rack cabinet is typically” 42U “in height, 19 inches (482.60 mm) wide, and 36 inches (914.40 mm) deep.” However, earlier, that same article (Wikipedia's article for “19-inch rack”, section called “Equipment mounting”, sub-section called “Structural support” says, “Although there is no standard for the depth of equipment, nor specifying the outer width and depth of the rack enclosure itself (incorporating the structure, doors and panels that contain the mounting rails), there is a tendency for 4-post racks to be 600 mm (23.62 in) or 800 mm (31.50 in) wide, and for them to be 600 mm (23.62 in), 800 mm (31.50 in) or 1,010 mm (39.76 in) deep.”

Wikipedia's article for “rack unit” notes, “When used to describe the rack enclosure itself, the term "half-rack" typically means a rack enclosure that is 22U tall.” This would imply that a “full rack” would be 44U tall, although that doesn't seem to be a strict standard. There are also 46U tall racks. Other sizes that appear to be common (based on some quick Google probing) could include 24U or 38U.

What may be more standardized are the widths of the racks, although there are multiple standards: the most common standard is 19 inches wide, although there is also a 23 inch variation that is frequent enough to be mentioned somewhat commonly.

Note that some of the measurements being used will commonly involve some rounding, or even approximation. Wikipedia's article for “19-inch rack”, section called “Overview and history” has stated, “The EIA standard was revised again in 1992” ... “making each "U" 44.50 millimetres (1.752 in).” Yet other places identify a U as 1.75 inches tall, which is 44.45mm tall. The difference of two millimeters per U would amount to 8.4 cm (about 3.23 inches) over 42U. There may also be a bit of incongruity as to whether a measurement refers to the front panel of a piece of equipment, or the center of mounting holes, etc. So, don't necessarily count on common terms as actually having a certain precise definition until you get such specific details sufficiently verified.

[#raid]: RAID

Now most commonly menaing “Redundant Array of Independent Disks”, this is an updated expansion of an acronym that originally stood for “Redundant Array of Inexpensive Disks”. Both phrases refer to the same technology. See: section about RAID.

For a similar acronym, see RAIE.

[#ram]: Random Access Memory (“RAM”)
Generally much faster than long term storage. Generally volatile, meaning that it needs ongoing electricity to reliably keep contents. Generally much slower than hardware cache, although also much more economical (for initial purchase), particularly for higher quantities.
Redundant Array of Independent Disks
Redundant Array of Inexpensive Disks
[#raie]: Redundant Array of Inexpensive Engineers (“RAIE”)

The term is clearly meant to be play on words, being similar to “Redundant Array of Inexpensive Disks”, which is the meaning of the acronym “RAID” used in the first major paper that described RAID.

This is a derogatory/derogative/disparaging term which might be used by people in wealthy nations who disapprove of IT support being sent to places that offer labor for lower costs, especially if that labor is supplied by a place that is far away (such as in India, which is overseas for Americans). The implication is that each individual inexpensive engineer is less capable, but that an organization is hoping to have quantity make up for having less quality in each individual person's talent. The way the organization intends to do this is by having a redundant array of numbskulls. How could anything possibly go wrong?

[#rarindpd]: Redundant Array of Independent Disks
[#rarinxpd]: Redundant Array of Inexpensive Disks
[#ripencc]: “Réseaux IP Européens Network Coordination Centre” (“RIPE NCC”)

An RIR. The name RIPE is French for “European IP Networks”, although non-European nations (most notably Russia) are handled by this RIR. See: Regional Internet Registries (section within documentation about IANA).

[#rir]: Regional Internet Registry

An organization that has received IP addresses (both IPv6 and IPv4 addresses) from IANA, for the purpose of then re-distributing them to other organizations such as service providers. Some of the early RIR's have included ARIN, RIPE-NCC, and APNIC.

See: Regional Internet Registries (section within documentation about IANA).

[#rirabbr]: RIR
See: Regional Internet Registry.
[#rj11]: RJ11 (a.k.a. “RJ-11”, RJ stands for “Registered Jack”)
The connector used for a standard telephone jack. (Frequently the Category of Cabling used for telephone cords is Cat3.)
[#rj45]: RJ-45 (a.k.a. “RJ45”, RJ stands for “Registered Jack”)

This is a term that is commonly used to refer to the type of connector used for connectors found at the end of standard Ethernet cables (that contain twisted pair copper wiring).

[#rj45usag]: (in)correctness of the usage of the term RJ-45

Wikipedia's article for “Modular connector”: section called “8P8C” notes, “Although commonly referred to as” RJ45, like found in Ethernet's “category 5 cables, it is incorrect to refer to a generic 8P8C connector as an RJ45.” There are a number of printed books quoted to back up that statement. The term RJ-45 referred to a wiring standard: Perhaps most commonly utilized was a standard called “RJ45S” which is not compatible (if for no other reason, simply because the connector differed). “The original RJ45S” was a standard used by telephone companies, but “is obsolete today.” Various sources have noted that the actual RJ45S standard involved a physical connector that was keyed and, because of this key, does not plug into a modern standard RJ-45 jack.

For instance, Wikipedia's article for ISO/IEC 11801, section on “Category 7”, e.g. from February 2021 states, “the usage” of the referred term “RJ45” “is somewhat confusing -- only the original telephone connector is properly "RJ-45", and those connectors, though mechanically interworkable, are not compatible with high speed digital use.” Being that the R in RJ stands for “registered”, it could make sense that equipment which does not follow the formal specification wouldn't be considered properly “registered”.

However, the term RJ-45 is widely used anyway. Wikipedia's article for “RJ45 (telecommunications)”, section called “RJ45” notes, “Virtually all electronic equipment which uses an 8P8C connector (or possibly any 8P connector at all) will document it as an "RJ45" connector.” RJ-45, and not 8P8C, is the more well-recognized term (even if it is being used in a technically incorrect manner). Wikipedia's article for “Registered jack”: “Registered jack types” notes, “The true RJ45(S) jack is rarely used, but the name RJ45 commonly refers to any 8P8C modular connector.”

So, in practice, the term ends up referred to the standard connectors used by equipment that utilizes Ethernet twisted pair cabling.

In general, there is no difference between using RJ-45 and RJ45. Wikipedia's article for “Registered jack” (section called “Registered jack types”) lists severla different types of registered jacks that start with “RJ” followed by a number, not a hyphen. So perhaps RJ followed by a hyphen will look a little different for anybody who is very familiar with the standards. “RJ-45” (with the hyphen) is probably a bit more common of a notation when referring to the standard used by computers.

Wikipedia's article for “RJ45 (telecommunications)”, section called “RJ45” notes, the “physical connector is standardised as the IEC 60603-7 8P8C modular connector”. “The physical dimensions of the male and female connectors are specified in ANSI/TIA-1096-A and ISO-8877 standards and normally wired to the T568A and T568B pinouts specified in the TIA/EIA-568 standard to be compatible with both telephone and Ethernet.”

[#rodent]: rodent

A “mouse-like” thing. Mice are rodents, and other devices that are rodents include trackballs, touchpads, pointing sticks (the “Classic Dome”/“eraser head” rodents, and other analog (joy)sticks), graphics/drawing tablets (or the input of a tablet PC when written on, such as with a stylus). Another rather common term, for this type of peripheral, is a “pointing device”.

Basically, the term refers to something that allows a user to quickly move a cursor from one location of a grid (such as a screen full of pixels) to another location.

The term has nearly fell into complete disuse, enough that it may not even be widely recognized by many in the computer industry. How sad! It's a great term.

With the advent of touch screens becoming commonly utilized, it is safely and clearly undeniable that, using the standard old definition, fingers are now able to be classified as rodents. So, people now generally have five rodents coming from the base of each of their hands.


[#router]: router

Before simplifying things, let's start off by looking at some official definitions:

Don't get too worried about RFC 2460's reference to “IPv6 packets”, because RFC 2460 is about IPv6. Basically, other network traffic simply wasn't intended to be considered relevant to RFC 2460.

To simplify this, routing refers to the idea of taking traffic from one (sub)network, and relaying a copy of that traffic onto another (sub)network. This is called “forwarding” a packet to another (sub)network. A device that does that is a router.

In contrast, a device (such as a network “switch”) that forwards traffic to another device, but only to a device that is on the same (sub)network, is not a router.

Similiar/related terms include: host (for a device that does not forward packets), and node (of a network).

For completeness's sake, a complication may be worth pointing out: Some people may have a slightly different definition of whether a device is officially classified to be a “router”. This gets discussed a bit further in the section about multi-layer switches. However, to simplify things, the description provided here discusses the usage that is recommended, in part because it is what is most understood (by many technicians in the field).

RFC 1812: section 2.2.3 (“Routers”) provides a longer (multi-paragraph) description of routers.

[#rpmterm]: RPM
Redhat Package Manager

Some older software was caled “RedHat Package Manager”. Newer versions have been renamed to “RPM Package Manager”, resulting in a “recursive acronym” which looks a bit less vendor-dependent.

Ranish Partition Manager

Software for DOS which could modify the boundaries of disk partitions using the MBR partition scheme. Many versions of DOS operating systems came with software called “FDISK” which could perform many of the functions that Ranish Partition Manager could, although those versions of “FDISK” were often far more limited.

[#rs232]: RS-232

A signaling standard that was often used by DE-9 ports and DB-25 ports (see: DB ports). Some systems could have both a DE-9 connector and a DB-25 connector, both of which could have a cable designed to be compatible with the same set of pins on a circuit board (an “I/O card”, or a motherboard). What the software was really designed to do was to work with the pins on the circuitry, without caring whether the attached cable went to a female DE-9 port or a male DB-25 port. Therefore, some software would refer to using the RS-232 standard, rather than referring to the physical connections.