The Web Hosting Tech Support Forum

[Julia]

Hello

We just received a regular RFQ for colocating a server.

So far all is fine until they wrote the last line:

"And for IP, a /28 and a /29. Also, we would need the /28 to be statically routed to one of the IP addresses in the /29."

What do you think they mean by this? What's a /28 and a /29?

Other than that, everything else looks pretty simple, I'd say.

Thanks

Julia

[Donovan B.]

Hello Julia

The reference you are seeing is called CIDR notation.

They are requesting two have two separate subnets.

Classless Inter-Domain Routing (CIDR, pronounced "cider") was introduced in 1993 and is the latest refinement to the way IP addresses are interpreted.

It replaced the previous generation of IP address syntax, classful networks. Specifically, rather than allocating address blocks on eight-bit (i.e., octet) boundaries forcing 8, 16, or 24-bit prefixes, it used the technique of variable-length subnet masking (VLSM) to allow allocation on arbitrary-length prefixes.

CIDR encompasses:

* The VLSM technique of specifying arbitrary length prefix boundaries.

A CIDR-compliant address is written with a suffix indicating the number of bits in the prefix length, such as 192.168.0.0/16. This permits more efficient use of increasingly scarce IPv4 addresses.

* The aggregation of multiple contiguous prefixes into supernets, and, wherever possible in the Internet, advertising aggregates, thus reducing the number of entries in the global routing table.

Aggregation hides multiple levels of subnetting from the Internet routing table, and reverses the process of "subnetting a subnet" with VLSM.

* The administrative process of allocating address blocks to organizations based on their actual and short-term projected need, rather than the very large or very small blocks required by classful addressing schemes.

IPv6 utilizes the CIDR convention of indicating prefix length with a suffix, but the longer address field of IPv6 made it unnecessary to practice great economy in allocating the minimum amount of address space an organization could justify. The concept of class was never used in IPv6.

CIDR is principally a bitwise, prefix-based standard for the interpretation of IP addresses. It facilitates routing by allowing blocks of addresses to be grouped together into single routing table entries.

These groups, commonly called CIDR blocks, share an initial sequence of bits in the binary representation of their IP addresses. IPv4 CIDR blocks are identified using a syntax similar to that of IPv4 addresses: a four-part dotted-decimal address, followed by a slash, then a number from 0 to 32:

A.B.C.D/N. The dotted decimal portion is interpreted, like an IPv4 address, as a 32-bit binary number that has been broken into four octets.

The number following the slash is the prefix length, the number of shared initial bits, counting from the left-hand side of the address.

When speaking in abstract terms, the dotted-decimal portion is sometimes omitted, thus a /20 is a CIDR block with an unspecified 20-bit prefix.

If you have any questions or comments, feel free to contact us.

Thank you.

The colocation support team,
Sun Hosting
http://www.sunhosting.ca/server-colocation-montreal.html

[Julia]

Ok, thanks for the answer. Is CIDR something that Sun Hosting can support?

[Donovan B.]

Julia,

I'm not exactly sure what you mean by if we can support CIDR.

To clarify our original response, "CIDR" or "Classless Inter-Domain Routing" is a way that IP addresses are interpreted.

This basically means that something like /24 also can be stated as one C class of IPs, or even stated as a grouping of 256 IP addresses in a block.

For a list of CIDR notation and their numerical value of ip addresses, please see below:

****** CIDR Prefix Aggregation ******

Another benefit of CIDR is the possibility of routing prefix aggregation (also known as "supernetting" or "route summarization"). For example, sixteen contiguous Class C (/24) networks could now be aggregated together, and advertised to the outside world as a single /20 route (if the first 20 bits of their network addresses match).

Two aligned contiguous /20s could then be aggregated to a /19, and so forth. This allows a significant reduction in the number of routes that have to be advertised over the Internet, preventing 'routing table explosions' from overwhelming routers, and stopping the Internet from expanding further.

CIDR IP/CIDR Δ to last IP addr Mask Hosts (*) Class Notes
a.b.c.d/32 +0.0.0.0 255.255.255.255 1 1/256 C
a.b.c.d/31 +0.0.0.1 255.255.255.254 2 1/128 C d = 0 ... (2n) ... 254
a.b.c.d/30 +0.0.0.3 255.255.255.252 4 1/64 C d = 0 ... (4n) ... 252
a.b.c.d/29 +0.0.0.7 255.255.255.248 8 1/32 C d = 0 ... (8n) ... 248
a.b.c.d/28 +0.0.0.15 255.255.255.240 16 1/16 C d = 0 ... (16n) ... 240
a.b.c.d/27 +0.0.0.31 255.255.255.224 32 1/8 C d = 0 ... (32n) ... 224
a.b.c.d/26 +0.0.0.63 255.255.255.192 64 1/4 C d = 0, 64, 128, 192
a.b.c.d/25 +0.0.0.127 255.255.255.128 128 1/2 C d = 0, 128
a.b.c.0/24 +0.0.0.255 255.255.255.000 256 1 C
a.b.c.0/23 +0.0.1.255 255.255.254.000 512 2 C c = 0 ... (2n) ... 254
a.b.c.0/22 +0.0.3.255 255.255.252.000 1,024 4 C c = 0 ... (4n) ... 252
a.b.c.0/21 +0.0.7.255 255.255.248.000 2,048 8 C c = 0 ... (8n) ... 248
a.b.c.0/20 +0.0.15.255 255.255.240.000 4,096 16 C c = 0 ... (16n) ... 240
a.b.c.0/19 +0.0.31.255 255.255.224.000 8,192 32 C c = 0 ... (32n) ... 224
a.b.c.0/18 +0.0.63.255 255.255.192.000 16,384 64 C c = 0, 64, 128, 192
a.b.c.0/17 +0.0.127.255 255.255.128.000 32,768 128 C c = 0, 128
a.b.0.0/16 +0.0.255.255 255.255.000.000 65,536 256 C = 1 B
a.b.0.0/15 +0.1.255.255 255.254.000.000 131,072 2 B b = 0 ... (2n) ... 254
a.b.0.0/14 +0.3.255.255 255.252.000.000 262,144 4 B b = 0 ... (4n) ... 252
a.b.0.0/13 +0.7.255.255 255.248.000.000 524,288 8 B b = 0 ... (8n) ... 248
a.b.0.0/12 +0.15.255.255 255.240.000.000 1,048,576 16 B b = 0 ... (16n) ... 240
a.b.0.0/11 +0.31.255.255 255.224.000.000 2,097,152 32 B b = 0 ... (32n) ... 224
a.b.0.0/10 +0.63.255.255 255.192.000.000 4,194,304 64 B b = 0, 64, 128, 192
a.b.0.0/9 +0.127.255.255 255.128.000.000 8,388,608 128 B b = 0, 128
a.0.0.0/8 +0.255.255.255 255.000.000.000 16,777,216 256 B = 1 A
a.0.0.0/7 +1.255.255.255 254.000.000.000 33,554,432 2 A a = 0 ... (2n) ... 254
a.0.0.0/6 +3.255.255.255 252.000.000.000 67,108,864 4 A a = 0 ... (4n) ... 252
a.0.0.0/5 +7.255.255.255 248.000.000.000 134,217,728 8 A a = 0 ... (8n) ... 248
a.0.0.0/4 +15.255.255.255 240.000.000.000 268,435,456 16 A a = 0 ... (16n) ... 240
a.0.0.0/3 +31.255.255.255 224.000.000.000 536,870,912 32 A a = 0 ... (32n) ... 224
a.0.0.0/2 +63.255.255.255 192.000.000.000 1,073,741,824 64 A a = 0, 64, 128, 192
a.0.0.0/1 +127.255.255.255 128.000.000.000 2,147,483,648 128 A a = 0, 128
0.0.0.0/0 +255.255.255.255 000.000.000.000 4,294,967,296 256 A

(*) Note that for routed subnets bigger than /31 or /32, 2 needs to be subtracted from the number of available addresses - the largest address is used as the broadcast address, and typically the smallest address is used to identify the network itself. See RFC 1812 for more detail. It is also common for the gateway IP for that subnet to use an address, meaning that you would subtract 3 from the number of usable hosts that can be used on the subnet.

Historical background

IP addresses were originally separated into two parts: the network address (which identified a whole network or subnet), and the host address (which identified a particular machine's connection or interface to that network). This division was used to control how traffic was routed in and among IP networks.

Historically, the IP address space was divided into three main 'classes of network', where each class had a fixed size network address. The class, and hence the length of the network address and the number of hosts on the network, could always be determined from the most significant bits of the IP address.

Without any way of specifying a prefix length or a subnet mask, routing protocols, such as RIP-1, IGRP, necessarily used the class of the IP address specified in route advertisements to determine the size of the routing prefixes to be set up in the routing tables.

As the experimental TCP/IP network expanded into the Internet during the early 1980s, the need for more flexible addressing schemes became increasingly apparent. This led to the successive development of subnetting and CIDR.

Because the old class distinctions are ignored, the new system was called classless routing. It is supported by modern routing protocols, such as RIP-2, EIGRP, IS-IS and OSPF. This led to the original system being called, by back-formation, classful routing.

Variable-Length Subnet Masking (VLSM) is the same concept as CIDR, but is mostly in historical usage.

Internet RFC 1338 was a major paradigm shift to establish a provider-based addressing and hierarchical routing. With the new RFC 1338-style provider-based supernetting, it was possible to create multiple hierarchical tiers and most tiers were envisioned to be internet service providers.

Provider-based address space allocation was the new model, and BGP would evolve to BGP4, incorporating the RFC 1338 paradigm. For this shift to occur, the technique for supernetting-subnetting the IP address space required a modification. This new feature was called Classless Inter-Domain Routing (CIDR). (Note that RFC 1338 was replaced by RFC 1519)

* RFC 1518 - An Architecture for IP Address Allocation with CIDR
* RFC 4632 - Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy
* RFC 2317 documents a way to do reverse DNS delegation for CIDR blocks
* RFC 4291 - IP Version 6 Addressing Architecture
* RFC 3021 - PtP links using /31 subnets (but /32 is more flexible)
* Online IP CIDR Calculator
* Freeware command-line subnet calculator for Windows and Linux
* Command-line calculator by Krischan Jodies (also in Debian)
* Grep-like command-line tool to match IP addresses by CIDR spec

Hope that helps

The colocation support group,
Sun Hosting
http://www.sunhosting.ca/colocation.html