Ex2 Chapter 6 - Classful/classless, CIDR and FLSM/VLSM

When the ARPANET project was commissioned in 1969 no one expected to turn out this successful and into what we now call the Internet.  As of January 2007, there are over 433 million hosts on the Internet.

1969 - ARPANET - Advanced Research Projects Agency Network - USA Department of Defense
1969 - RFC (Request for Comments) establishing
1971 - FTP (RFC 114)
1972 - First E-Mail protocols released
1981 - SMTP (RFC 788)
1981 - IPv4 Classfull  (RFC 791) + ICMPv4 (RFC 792)
1981 - TCP (RFC 793) + 1980 UDP (RFC 768)
1983 January 1 - transition of the ARPANET host protocol from NCP to TCP/IP
1983 - Domain Name System (DNS)
1988 - OSI Reference Model released
1989 - Border Gateway Protocol (BGP)
1991 - WWW
1993 - VLSM and CIDR – IETF introduced (RFC 1519)
1994 - NAT
1996 - Private Addressing (RFC 1918)


The following technologies have allowed IPv4 to not be exhausted:
•  VLSM and CIDR – IETF introduced in 1993 (RFC 1519)
•  NAT – 1994 (RFC 1631)
•  Private Addressing – 1996 (RFC 1918)

Classfull IPv4 Addressing
In the original specification of IPv4, the authors established the classes to provide three different sizes of networks for large, medium, and small organizations.
Address Formats  ( RFC791 [Page 24]  )
HighOrderBits   Format                  Class  Meaning                          Ranges
---------   --------------------------- -----  --------                      ----------------
0            7 bits net, 24 bits host       IP starting with 0xxx'xxxx.x  0.0.0.0 (00000000)-127.255.255.255 (01111111)
10          14 bits net, 16 bits host       IP starting with 10xx'xxxx.x  128.0.0.0 (10000000)-191.255.255.255 (10111111)
110         21 bits net,  8 bits host       IP starting with 110x'xxxx.x  192.0.0.0 (11000000)-223.255.255.255 (11011111)

111         escape to extended addressing mode IP starting with 111x'xxxx.x
1110        multicast                       IP starting with 1110'xxxx.x    224.0.0.0 - 239.255.255.255
1111        Reserved-experimental&future    IP starting with 1111'xxxx.x    240.0.0.0 - 255.255.255.255

0.0.0.0/8 - reserved (A value of zero in the network field means this network).
127.0.0.0/8 - reserved for loopback testing.
The remaining addresses were reserved for multicasting and future uses:
Class D - Multicast addresses begin with 1110 - Multicast addresses are used to identify a group of hosts that are part of a multicast group. This helps reduce the amount of packet processing that is done by hosts, particularly on broadcast media (i.e., Ethernet LANs). Routing protocols, such as RIPv2, EIGRP, and OSPF use designated multicast addresses (RIP = 224.0.0.9, EIGRP = 224.0.0.10, OSPF 224.0.0.5, and 224.0.0.6).
Class E - Reserved IP addresses begin with 1111 - These addresses were reserved for experimental and future use.

Classful networks:
Class A  -  128 networks (2 reserved)  with 16,777,214 host each
Class B  -  16,348 nets with 65,534 hosts each
Class C  -  2,097,152 nets with 254 hosts each

Classful Subnet Mask
Using classful IP addresses meant that the subnet mask of a network address could be determined by the value of the first octet, or more accurately, the first three bits of the address.
Routing protocols, such as RIPv1, only need to propagate the network address of known routes and do not need to include the subnet mask in the routing update.

As specified in RFC 790, each network class has a default subnet mask associated with it:
Class A 255.0.0.0
Class B - 255.255.0.0
Class C - 255.255.255.0
An advantage of assigning specific default subnet masks to each class is that it made routing update messages smaller.
Classful routing protocols do not include the subnet mask information in their updates. The receiving router applies the default mask based on the value of the first octet which identifies the class.

Router receiving the routing update determining the subnet mask:
 - simply by examining the value of the first octet in the network address,
 - or by applying its ingress interface mask for subnetted routes. The subnet mask was directly related to the network address.

Between routers with same subnet mask on interface, network updates are sent as 172.16.2.0  (/24 fo example).
Between router have to announce (172.16.2.0/24) to another router, but 192.168.1.0/24 is between them, 172.16.2.0/ (24) will be announced as 172.16.0.0 (CLASSFUL B - on boundaries of MAJOR NETWORKS).

Auto summarization - Router will summarize routes to their classful networks between major networks, i.e; class a, b, c 
RIP performs auto-summarization on the classful network boundary every time the major/classful advertised network is different from the major network of the interface to which the updates are sent! - See more at: http://www.costiser.ro/2013/09/13/rip-auto-summary-and-discontiguous-networks/#sthash.DUuTtzH4.dpuf
RIP performs auto-summarization on the classful network boundary every time the major/classful advertised network is different from the major network of the interface to which the updates are sent! - See more at: http://www.costiser.ro/2013/09/13/rip-auto-summary-and-discontiguous-networks/#sthash.DUuTtzH4.dpuf
RIP performs auto-summarization on the classful network boundary  - every time the major/classful advertised network is different from the major network of the interface to which the updates are sent!
Some other notes about this:
  • auto-summary is enabled by default
  • it does not install a Null0 for the auto-summaries that it generates
  • RIPv2 allows turning off the automatic summarization, but RIPv1 does not have this capability
- See more at: http://www.costiser.ro/2013/09/13/rip-auto-summary-and-discontiguous-networks/#sthash.DUuTtzH4.dpuf
 Some other notes about this:
 - auto-summary is enabled by default
 - it does not install a Null0 for the auto-summaries that it generates
 - RIPv2 allows turning off the automatic summarization, but RIPv1 does not have this capability

RIP performs auto-summarization on the classful network boundary every time the major/classful advertised network is different from the major network of the interface to which the updates are sent! - See more at: http://www.costiser.ro/2013/09/13/rip-auto-summary-and-discontiguous-networks/#sthash.DUuTtzH4.dpuf
CIDR
In 1993, IETF introduced Classless Inter-domain Routing (CIDR), which allows:
•  More efficient use of IPv4 address space.
•  Prefix aggregation, which reduces the size of routing tables.

Routers that use CIDR ignore classful boundaries.  In cases where CIDR is used the terminology is that a supernet has been created compared to a subnet.  Classless routing protocols must be used to allow this technology to be propagated throughout the network.

Variable Length Subnet Masking (VLSM) is used by CIDR to achieve it’s results. 
Another term for route summarization is route aggregation.  

A supernet is always a route summary but a route summary is not always a supernet.

The overall result was that the classful addressing was a very wasteful addressing scheme.
A better network addressing solution had to be developed. For this reason, Classless Inter-Domain Routing (CIDR) was introduced in 1993.

IETF introduced CIDR in RFC 1517 in 1993.
CIDR replaced the classful network assignments and address classes (A, B, and C) became obsolete. Using CIDR, the network address is no longer determined by the value of the first octet.
Classless Inter-Domain Routing (CIDR) is a methodology of allocating IP addresses and routing Internet Protocol packets.
Classless Inter-Domain Routing is based on variable-length subnet masking (VLSM) to allow Network to be divided to different-size subnets . This allows efficient use of subnets and avoids wasting IP addresses. Variable-length subnet masks are mentioned in RFC 950 (1985).

CIDR also reduces the size of routing tables and manages the IPv4 address space more efficiently using:
- Route summarization - Also known as prefix aggregation, routes are summarized into a single route to help reduce the size of routing tables. For instance, one summary static route can replace several specific static route statements.
- Supernetting - Occurs when the route summarization mask is a smaller value than the default traditional classful mask.

route summary: 192.168.1.0/26 = 192.168.1.0/28 + 192.168.1.16/28 + 192.168.1.32/28 + 192.168.1.48/16
supernet     : 192.168.0.0/23 (/23 - is smaller than classful /24 Class C)

Note: A supernet is always a route summary, but a route summary is not always a supernet.

Summary routes can be configured by both static routes and classless routing protocols.
ClassFUL routing protocols cannot send supernet routes.
Note: When a supernet route is in a routing table, for example, as a static route, a classful routing protocol does not include that route in its updates.
Propagating VLSM and supernet routes requires a classless routing protocol such as RIPv2, OSPF, or EIGRP. Classless routing protocols advertise network addresses with their associated subnet masks.

VLSM
With fixed-length subnet masking (FLSM), the same number of addresses is allocated for each subnet.
Note: FLSM is also referred to as traditional subnetting.
In traditional subnetting the same subnet mask is applied for all the subnets. This means that each subnet has the same number of available host addresses.

With VLSM the subnet mask length varies depending on how many bits have been borrowed for a particular subnet, thus the “variable” part of variable-length subnet mask.

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