Tinh CIDR ( Supernet )

[telecom1988]

Supernetting

Giả sử ta mạng của ta có 3 Subnets:
Accounting: gồm 200 hosts
Finance : gồm 400 hosts
Marketing: gồm 200 hosts
Bạn hòa mạng với Internet và được Internet Service Provider (ISP) cho 4 Class C IP addresses như sau:

192.168.9.0/24
192.168.10.0/24
192.168.11.0/24
192.168.12.0/24

Bạn có 3 segments và bạn muốn mỗi segment chứa một Network.
Bây giờ bạn làm sao?
Địa chỉ IP trong Class C với default subnet mask 24 cho ta con số Hosts tối đa trong mỗi Network là [(2^X) – 2] = (2^ – 2 = 254. Như thế segments Accounting và Marketing không bị trở ngại nào cả.
Nhưng ta thấy Segment Finance cần thêm 1 bit mới đủ. Ta làm như sau:
Bước 1: Liệt kê Network IP addresses trong dạng nhị phân

192.168.9.0/24 11000000 10101000 00001001 00000000 (1)
192.168.10.0/24 11000000 10101000 00001010 00000000 (2)
192.168.11.0/24 11000000 10101000 00001011 00000000 (3)
192.168.12.0/24 11000000 10101000 00001100 00000000 (4)

Bước 2: Nhận diện network prefix notation

23 bits đầu (từ trái qua phải) của 2 network IP address (2) and (3) đều giống nhau.
Nếu chúng ta thu Subnet mask từ 24 xuống 23 cho (2) và (3) ta sẽ có một Subnet có thể cung cấp 508 hosts.
IP address của mỗi segment trở thành:

Accounting: gồm 200 hosts: 192.168.9.0/24
Finance: gồm 400 hosts: 192.168.10.0/23
Marketing: gồm 200 hosts: 192.168.12.0/24

Bây giờ IP address 192.168.11.0 trở thành một HostID tầm thường trong Subnet 192.168.10.0/23.

Quá trình ta làm vừa qua bằng cách bớt số bits trong Subnet mask khi gom hai hay bốn (v.v..) subnets lại với nhau để tăng con số HostID tối đa trong một Subnet, được gọi là SUPERNETTING

[telecom1988]

Summay và CIDR giống nhau điều là tóm tắt.
Khác nhau:
-Summary: chỉ được tóm tắt với subnetmask lớn hơn hoặc bằng subnet của major.
-CIDR: có thể tóm tắt route với subnetmask nhỏ hơn subnetmask của major.

Các giao thức classless thì hỗ trợ CIDR: RIPv2, EIGRP, OSPF, BGP. Vì classless có mang theo subnetmask.
các giao thức classfull không hỗ trợ CIDR: RIPv1, IGRP.

Riêng đối với giao thức linkstate như OSPF thì do không quảng bá route [Trong vùng chỉ quảng bá trạng thái link] nên chỉ làm summary dạng CIDR ngay tại những con có khả năng quảng bá route như ABR, ASBR.

Đối Với RIPv2 mặc dầu nói có hỗ trợ CIDR nhưng có lẽ cisco thì không. Nếu lúc bạn đánh summary theo dạng CIDR thì nó báo thế này.
vd:
R1(config-if)#ip summary-address rip 172.0.0.0 255.224.0.0
Summary mask must be greater or equal to major net

-Summary theo CIDR thì không cần gõ thêm no-autosum vì lệnh này không liên quan gì tới nó cả. Lệnh này chỉ dùng khi ta dùng VLSM mà muốn giao thức ta dùng quảng bá chính xác route chứ không tự động summary ngay ranh giới.
__________________

[telecom1988]

Summary bình thường thì chỉ summay lớn nhất là mạng major của nó.
vd: 192.168.1.64/26 và 192.168.1.128/26, 192.168.1.0/26 summary lại thì sao?

Có phải nó là 192.168.1.0/24 không?
Hay 192.168.1.0/23 cũng được vì mạng này là cha của 192.168.1.0/24.


Bình thường thì summary chỉ cho summary ngay tại major là hết rồi, tức là 192.168.1.0/24 [đó là summary theo dạng thông thường].

Còn bạn muốn summary những mạng lớn hơn hoặc bằng major thì giao thức đó phải hỗ trợ CIDR.
vd: 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24
Summary lại có phải là 192.168.0.0/22 không?
Nhưng đối với những giao thức không hỗ trợ CIDR thì nó không cho bạn làm như vậy. Bởi vì /22 thì nhỏ hơn major /24 [24 do 192.x.x.x là lớp C]
Còn đối với giao thức hỗ trợ CIDR thì cho bạn summary sao tùy ý ngay cả khi lớp mạng bạn summary lớn hơn major. [/22]

[telecom1988]

During the first decade of the modern Internet after the invention of the Domain Name System (DNS) it became apparent that the devised system based on classful network design of distributing the address space and routing IP packets was not scalable (cf. RFC 1517). To alleviate the shortcomings, the Internet Engineering Task Force published in 1993 a new set of standards, RFC 1518 and RFC 1519, to define a new concept of allocation of IP address blocks and new methods of routing IPv4 packets. RFC 1519 was replaced by RFC 4632 in 2006.

An IP address is interpreted in two parts: a network-identifying prefix followed by a host address within that network. In the prior classful network architecture IP address allocations were based on octet (8-bit) boundary segments of the 32-bit IP address, forcing either 8, 16, or 24-bit network prefixes. Thus, the smallest allocation and routing block contained only 256 addresses—too small for most enterprises, and the next larger block contained 65,536 addresses—too large to be used efficiently by even large organizations. This led to inefficiencies in address use as well as routing because the large number of allocated small (class-C) networks with individual route announcements, being geographically dispersed with little opportunity for route aggregation, created heavy demand on routing equipment.

Classless Inter-Domain Routing is based on variable-length subnet masking (VLSM) to allow allocation on arbitrary-length prefixes. Variable-length subnet masks are mentioned in RFC 950 (1985).

CIDR encompasses:

the VLSM technique of specifying arbitrary-length prefixes. An address in CIDR notation is written with a suffix indicating the number of bits in the prefix, 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.
While IPv6 maintains the IPv4 CIDR convention of indicating prefix length with a suffix, the IPv4 concept of class was abandoned in IPv6.

[telecom1988]

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.

An IP address is part of a CIDR block, and is said to match the CIDR prefix if the initial N bits of the address and the CIDR prefix are the same. Thus, understanding CIDR requires that IP address be visualized in binary. Since the length of an IPv4 address is fixed at 32 bits, an N-bit CIDR prefix leaves 32 − N bits unmatched, and there are 2(32 − N) possible combinations of these bits, meaning that 2(32 − N) IPv4 addresses match a given N-bit CIDR prefix. Shorter CIDR prefixes match more addresses, while longer CIDR prefixes match fewer. An address can match multiple CIDR prefixes of different lengths.

CIDR is also used with IPv6 addresses, for which the prefix length can range from 0 to 128, due to the larger number of bits in the address. A similar syntax is used: the prefix is written as an IPv6 address, followed by a slash and the number of significant bits in the prefix mask

[telecom1988]

The Internet Assigned Numbers Authority (IANA) issues to Regional Internet Registries (RIRs) large, short-prefix CIDR blocks. For example, 62.0.0.0/8, with over sixteen million addresses, is administered by RIPE NCC, the European RIR. The RIRs, each responsible for a single, large, geographic area (such as Europe or North America), then subdivide these blocks into smaller blocks and issue them publicly. This subdividing process can be repeated several times at different levels of delegation. Large Internet service providers (ISPs) typically obtain CIDR blocks from an RIR, then subdivide them into smaller CIDR blocks for their subscribers, sized according to the size of the subscriber's network. Networks served by a single ISP are encouraged by IETF to obtain IP address space directly from their ISP. Networks served by multiple ISPs, on the other hand, will often obtain independent CIDR blocks directly from the appropriate RIR.

For example, in the late 1990s, the IP address 208.130.29.33 (since reassigned) was used by www.freesoft.org. An analysis of this address identified three CIDR prefixes. 208.128.0.0/11, a large CIDR block containing over 2 million addresses, had been assigned by ARIN (the North American RIR) to MCI. Automation Research Systems, a Virginia VAR, leased an Internet connection from MCI and was assigned the 208.130.28.0/22 block, capable of addressing just over 1000 devices. ARS used a /24 block for its publicly accessible servers, of which 208.130.29.33 was one.

All of these CIDR prefixes would be used, at different locations in the network. Outside of MCI's network, the 208.128.0.0/11 prefix would be used to direct to MCI traffic bound not only for 208.130.29.33, but also for any of the roughly two million IP addresses with the same initial 11 bits. Within MCI's network, 208.130.28.0/22 would become visible, directing traffic to the leased line serving ARS. Only within the ARS corporate network would the 208.130.29.0/24 prefix have been used.

[telecom1988]

A subnet mask is a bitmask that encodes the prefix length in a form similar to an IP address: 32 bits, starting with a number of 1 bits equal to the prefix length, ending with 0 bits, and encoded in four-part dotted-decimal format. A subnet mask encodes the same information as a prefix length, but predates the advent of CIDR.

CIDR uses variable-length subnet masks (VLSM) to allocate IP addresses to subnets according to individual need, rather than some general network-wide rule. Thus the network/host division can occur at any bit boundary in the address. The process can be recursive, with a portion of the address space being further divided into even smaller portions, through the use of masks which cover more bits.

CIDR/VLSM network addresses are now used throughout the public Internet, although they are also used elsewhere, particularly in large private networks. An average desktop LAN user generally does not see them in practice, as their LAN is usually numbered using special private network addresses

[telecom1988]

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.

See IPv4 subnetting reference.



http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

[telecom1988]

Netmask Netmask (binary) CIDR Notes
_____________________________________________________________________________
255.255.255.255 11111111.11111111.11111111.11111111 /32 Host (single addr)
255.255.255.254 11111111.11111111.11111111.11111110 /31 Unuseable
255.255.255.252 11111111.11111111.11111111.11111100 /30 2 useable
255.255.255.248 11111111.11111111.11111111.11111000 /29 6 useable
255.255.255.240 11111111.11111111.11111111.11110000 /28 14 useable
255.255.255.224 11111111.11111111.11111111.11100000 /27 30 useable
255.255.255.192 11111111.11111111.11111111.11000000 /26 62 useable
255.255.255.128 11111111.11111111.11111111.10000000 /25 126 useable
255.255.255.0 11111111.11111111.11111111.00000000 /24 "Class C" 254 useable

255.255.254.0 11111111.11111111.11111110.00000000 /23 2 Class C's
255.255.252.0 11111111.11111111.11111100.00000000 /22 4 Class C's
255.255.248.0 11111111.11111111.11111000.00000000 /21 8 Class C's
255.255.240.0 11111111.11111111.11110000.00000000 /20 16 Class C's
255.255.224.0 11111111.11111111.11100000.00000000 /19 32 Class C's
255.255.192.0 11111111.11111111.11000000.00000000 /18 64 Class C's
255.255.128.0 11111111.11111111.10000000.00000000 /17 128 Class C's
255.255.0.0 11111111.11111111.00000000.00000000 /16 "Class B"

255.254.0.0 11111111.11111110.00000000.00000000 /15 2 Class B's
255.252.0.0 11111111.11111100.00000000.00000000 /14 4 Class B's
255.248.0.0 11111111.11111000.00000000.00000000 /13 8 Class B's
255.240.0.0 11111111.11110000.00000000.00000000 /12 16 Class B's
255.224.0.0 11111111.11100000.00000000.00000000 /11 32 Class B's
255.192.0.0 11111111.11000000.00000000.00000000 /10 64 Class B's
255.128.0.0 11111111.10000000.00000000.00000000 /9 128 Class B's
255.0.0.0 11111111.00000000.00000000.00000000 /8 "Class A"

254.0.0.0 11111110.00000000.00000000.00000000 /7
252.0.0.0 11111100.00000000.00000000.00000000 /6
248.0.0.0 11111000.00000000.00000000.00000000 /5
240.0.0.0 11110000.00000000.00000000.00000000 /4
224.0.0.0 11100000.00000000.00000000.00000000 /3
192.0.0.0 11000000.00000000.00000000.00000000 /2
128.0.0.0 10000000.00000000.00000000.00000000 /1
0.0.0.0 00000000.00000000.00000000.00000000 /0 IP space

Net Host Total
Net Addr Addr Addr Number
Class Range NetMask Bits Bits of hosts
----------------------------------------------------------
A 0-127 255.0.0.0 8 24 16777216 (i.e. 114.0.0.0)
B 128-191 255.255.0.0 16 16 65536 (i.e. 150.0.0.0)
C 192-254 255.255.255.0 24 8 256 (i.e. 199.0.0.0)
D 224-239 (multicast)
E 240-255 (reserved)
F 208-215 255.255.255.240 28 4 16
G 216/8 ARIN - North America
G 217/8 RIPE NCC - Europe
G 218-219/8 APNIC
H 220-221 255.255.255.248 29 3 8 (reserved)
K 222-223 255.255.255.254 31 1 2 (reserved)
(ref: RFC1375 & http://www.iana.org/assignments/ipv4-address-space )
( http://www.iana.org/numbers.htm )
----------------------------------------------------------

The current list of special use prefixes:
0.0.0.0/8
127.0.0.0/8
192.0.2.0/24
10.0.0.0/8
172.16.0.0/12
192.168.0.0/16
169.254.0.0/16
all D/E space
(ref: RFC1918 http://www.rfc-editor.org/rfc/rfc1918.txt )
( or ftp://ftp.isi.edu/in-notes/rfc1918.txt )
(rfc search: http://www.rfc-editor.org/rfcsearch.html )
( http://www.ietf.org/ietf/1id-abstracts.txt )
( http://www.ietf.org/shadow.html )


Martians: (updates at: www.iana.org/assignments/ipv4-address-space )
no ip source-route
access-list 100 deny ip host 0.0.0.0 any
deny ip 0.0.0.0 0.255.255.255 any log ! antispoof
deny ip 0.0.0.0 0.255.255.255 0.0.0.0 255.255.255.255 ! antispoof
deny ip any 255.255.255.128 0.0.0.127 ! antispoof
deny ip host 0.0.0.0 any log ! antispoof
deny ip host [router intf] [router intf] ! antispoof
deny ip xxx.xxx.xxx.0 0.0.0.255 any log ! lan area
deny ip 0/8 0.255.255.255 any log ! IANA - Reserved
deny ip 1/8 0.255.255.255 any log ! IANA - Reserved
deny ip 2/8 0.255.255.255 any log ! IANA - Reserved
deny ip 5/8 0.255.255.255 any log ! IANA - Reserved
deny ip 7/8 0.255.255.255 any log ! IANA - Reserved
deny ip 10.0.0.0 0.255.255.255 any log ! IANA - Private Use
deny ip 23/8 0.255.255.255 any log ! IANA - Reserved
deny ip 27/8 0.255.255.255 any log ! IANA - Reserved
deny ip 31/8 0.255.255.255 any log ! IANA - Reserved
deny ip 36-37/8 0.255.255.255 any log ! IANA - Reserved
deny ip 39/8 0.255.255.255 any log ! IANA - Reserved
deny ip 41-42/8 0.255.255.255 any log ! IANA - Reserved
deny ip 50/8 0.255.255.255 any log ! IANA - Reserved
deny ip 58-60/8 0.255.255.255 any log ! IANA - Reserved
deny ip 69-79/8 0.255.255.255 any log ! IANA - Reserved
deny ip 82-95/8 0.255.255.255 any log ! IANA - Reserved
deny ip 96-126/8 0.255.255.255 any log ! IANA - Reserved
deny ip 127/8 0.255.255.255 any log ! IANA - Reserved
deny ip 169.254.0.0 0.0.255.255 any log ! link-local network
deny ip 172.16.0.0 0.15.255.255 any log ! reserved
deny ip 192.168.0.0 0.0.255.255 any log ! reserved
deny ip 192.0.2.0 0.0.0.255 any log ! test network
deny ip 197/8 0.255.255.255 any log ! IANA - Reserved
deny ip 220/8 0.255.255.255 any log ! IANA - Reserved
deny ip 222-223/8 0.255.255.255 any log ! IANA - Reserved
deny ip 224.0.0.0 31.255.255.255 any log ! multicast
deny ip 224.0.0.0 15.255.255.255 any log ! unless MBGP-learned routes
deny ip 224-239/8 0.255.255.255 any log ! IANA - Multicast
deny ip 240-255/8 0.255.255.255 any log ! IANA - Reserved

filtered source addresses
0/8 ! broadcast
10/8 ! RFC 1918 private
127/8 ! loopback
169.254.0/16 ! link local
172.16.0.0/12 ! RFC 1918 private
192.0.2.0/24 ! TEST-NET
192.168.0/16 ! RFC 1918 private
224.0.0.0/4 ! class D multicast
240.0.0.0/5 ! class E reserved
248.0.0.0/5 ! reserved
255.255.255.255/32 ! broadcast

ARIN administrated blocks: (http://www.arin.net/regserv/IPStats.html)
24.0.0.0/8 (portions of)
63.0.0.0/8
64.0.0.0/8
65.0.0.0/8
66.0.0.0/8
196.0.0.0/8
198.0.0.0/8
199.0.0.0/8
200.0.0.0/8
204.0.0.0/8
205.0.0.0/8
206.0.0.0/8
207.0.0.0/8
208.0.0.0/8
209.0.0.0/8
216.0.0.0/8
----------------------------------------------------------

well known ports: (rfc1700.txt)
www.iana.org/assignments/port-numbers

protocol numbers:
www.iana.org/assignments/protocol-numbers
www.iana.org/numbers.htm

ICMP(Types/Codes)
Testing Destination Reachability & Status
(0/0) Echo-Reply
(8/0) Echo
Unreachable Destinations
(3/0) Network Unreachable
(3/1) Host Unreachable
(3/2) Protocol Unreachable
(3/3) Port Unreachable
(3/4) Fragmentaion Needed and DF set (Pkt too big)
(3/5) Source Route Failed
(3/6) Network Unknown
(3/7) Host Unknown
(3/9) DOD Net Prohibited
(3/10) DOD Host Prohibited
(3/11) Net TOS Unreachable
(3/12) Host TOS Unreachable
(3/13) Administratively Prohibited
(3/14) Host Precedence Unreachable
(3/15) Precedence Unreachable
Flow Control
(4/0) Source-Quench [RFC 1016]
Route Change Requests from Gateways
(5/0) Redirect Datagrams for the Net
(5/1) Redirect Datagrams for the Host
(5/2) Redirect Datagrams for the TOS and Net
(5/3) Redirect Datagrams for the TOS and Host
Router
(6/-) Alternate-Address
(9/0) Router-Advertisement
(10/0) Router-Solicitation
Detecting Circular or Excessively Long Routes
(11/0) Time to Live Count Exceeded
(11/1) Fragment Reassembly Time Exceeded
Reporting Incorrect Datagram Headers
(12/0) Parameter-Problem
(12/1) Option Missing
(12/2) No Room for Option
Clock Synchronization and Transit Time Estimation
(13/0) Timestamp-Request
(14/0) Timestamp-Reply
Obtaining a Network Address (RARP Alternative)
(15/0) Information-Request
(16/0) Information-Reply
Obtaining a Subnet Mask [RFC 950]
(17/0) Address Mask-Request
(18/0) Address Mask-Reply
Other
(30/0) Traceroute
(31/0) Conversion-Error
(32/0) Mobile-Redirect

Ref: [RFC 792] [RFC 896] [RFC 950] [RFC 1016]
www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_5_3/cofigide/qos.htm#19774



Decimal system Prefix's
Factor Exponent Prefix
---------------------------------------------------
1 000 000 000 000 000 000 000 000...10^24....yotta
1 000 000 000 000 000 000 000...10^21....zetta
1 000 000 000 000 000 000...10^18....exa
1 000 000 000 000 000...10^15....peta
1 000 000 000 000...10^12....tera
1 000 000 000...10^9.....giga
1 000 000...10^6.....mega
1 000...10^3.....kilo
100...10^2.....hecto
10...10^1.....deka
0.1...10^-1....deci
0.01...10^-2....centi
0.001...10^-3....milli
0.000 001...10^-6....micro
0.000 000 001...10^-9....nano
0.000 000 000 001...10^-12...pico
0.000 000 000 000 001...10^-15...femto
0.000 000 000 000 000 001...10^-18...atto
0.000 000 000 000 000 000 001...10^-21...zepto
0.000 000 000 000 000 000 000 001...10^-24...yocto
---------------------------------------------------

Convert Fahrenheit <> Celsius:
Celsius = (Fahrenheit - 32) / 1.8
Fahrenheit = (Celsius * 1.8) + 32


last updated: 4jul02

[telecom1988]

http://en.wikipedia.org/wiki/Subnetwork

[telecom1988]

về subnet, thì có 2 "kiểu" có thể hiểu như thế này:
- kiểu 1: 1 classfull subnet chia thành nhiều subnet "nhỏ" hơn: khái niệm này gọi là subnetting,
- kiểu 2: "gộp" nhiều classfull subnet thành 1 subnet "bự" hơn: khái niệm này gọi là supernetting or CIDR
note: classfull subnet là những subnet thuộc các lớp A,B,C chưa bị chia hoặc gộp
trong ví dụ của bạn là minh họa khái niệm CIDR or Suppernetting, bạn thấy 192.x.x.x thuộc lớp classfull là lớp C, như vậy default-subnetmask của lớp C là /24. subnet 192.168.48.0/21 là gộp của 8 classfull subnet sau:
192.168.48.0/24
192.168.49.0/24
192.168.50.0/24
192.168.51.0/24
192.168.52.0/24
192.168.53.0/24
192.168.54.0/24
192.168.55.0/24

một ví dụ về subnetting:: classfull subnet 192.168.10.0/24 chia thành 4 subnets con sau:
192.168.10.0/25
192.168.10.128/26
192.168.10.192/27
192.168.10.224/27

note thêm 1 điểm nữa:
khái niệm chia subnet theo VLSM nghĩa là bạn chia subnet "cha" thành nhiều subnet "con" nhưng các subnet "con" có độ dài subnetmask khác nhau gọi là VLSM, ví dụ trên là chia theo VLSM
ngược với VLSM là FLSM (Fix Lengh Subnet MasK) nghĩa độ dài subnetmask của các subnet trọng mạng là bằng nhau

--------------------------------------------------------------------------------

[telecom1988]

IP Addressing and Subnetting

Address: The unique number ID assigned to one host or interface in a network
Subnet: A portion of a network sharing a particular subnet address
Subnet mask: A 32-bit combination used to describe which portion of an address refers to the subnet and which part refers to the host
Interface: A network connection.

RFC 1918
Understanding IP Addresses
32 binary bits(a network portion & host portion) = 4 octets (1 octet = 8 bits)
each octet is converted to decimal and separated by a period ((dot))
172.16.81.100
The value in each octet ranges from 0 to 255 decimal (00000000 --> 11111111)

1 1 1 1 1 1 1 1
128 64 32 16 8 4 2 1
(128+64+32+16+8+4+2+1=255)

0 1 0 0 0 0 0 1
0 64 0 0 0 0 0 1
(0+64+0+0+0+0+0+1=65)

10. 1. 23. 19 (decimal)
00001010.00000001.00010111.0001001 (binary)

classless interdomain routing (CIDR)

Class A (24 bits(nodes Id)) 1.0.0.0 - 127.255.255.255
Class B (16 bits( node id)) 128.0.0.0 - 191.255.255.255
Class C ( 8 bits( node id)) 192.0.0.0 - 223.255.255.255
Class D (Multicast Group ID (28 bits) 224.0.0.0 - 239.255.255.255
Class E (Experimental 27 bits) 240.0.0.0 - 254.255.255.255

Class A address are used for networks that have more than 65,536 hosts ( up to 16666214 hosts)
has a major network address of 10.
Class B address (256 and 65534 hosts)
has a major network address of 172.16
Class C address (less than 254 hosts)
has a major network address of 193.18.9

Network Masks
which portion of the address identifies the network
which portion of the address identifees the node
natuaral masks:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
A IP address on a Class A network
8.20.15.1 = 00001000.00001111.00000001
255.0.0.0 = 11111111.00000000.00000000

Understanding Subnetting
Subnetting ==> to create multiple logical networks that exist within a single Class A, B or C network.
Class C network
204.17.5.0 = 11001100.00010001.00000101.00000000
255.255.255.224 = 11111111.11111111.11111111.11100000
==> sub = 3 bits from the original host portion of the address and used them to make subnets
with these three bits, it is possible to create 8 subnets
with the remaining 5 host ID bits
each subnet can have up to 32 host addresses
30 of which can actually be assigned to a device
==<<>> These subnets have been created

204.17.5.0 255.255.255.224
host address range 1 to 30
204.17.5.32 255.255.255.224
host address range 33 to 62
204.17.5.64 255.255.255.224
host address range 65 to 94
204.17.5.96 255.255.255.224
host address range 97 to 126
204.17.5.128 255.255.255.224
host address range 129 to 158
204.17.5.160 255.255.255.224
host address range 161 to 190
204.17.5.192 255.255.255.224
host address range 193 to 222
204.17.5.224 255.255.255.224
host address range 225 to 254

2 ways: *3-bit subnet mask
the mask of 255.255.255.224 ==>> /27 --> 27bits are set in the mask
*CIDR = the notation prefix/length
204.17.5.32/27 denotes the network 204.17.5.32 255.255.255.224

The more host bits you use for a subnet mask
==>The more subnets you have available
The more subnets available
==> The less host addresses available per subnet
A Class C network of 204.17.5.0 255.255.255.224(/27)
==> 8 subnets
==> each with 32 host addresses (30 of which could be assigned to devices)
A Class C network of 204.17.5.0 255.255.255.240(/28)
204.17.5.0 11001100.00010001.00000101.00000000
255.255.255.240 11111111.11111111.11111111.11110000
---> 4 bits to make subnets with, ==> 4 bits left for host address.
==> 16 subnets
==> each of which have up to 16 host addresses (14 of which can be assigned to devices)



Class B network
172.16.0.0
its natural mask is 255.255.0.0 or 172.16.0.0/16
use a mask of 255.255.248.0 (/21)
how many subnets and hosts per subnet?

172.16.0.0 10101100.00010000.00000000.00000000
255.255.248.0 11111111.11111111.11111000.00000000

==>> 5 bits from the original host bits for subnets
-----> 32 subnets
===> 11 bits for host addresses
===> each subnet have 2048 host addresses
===> 2046 of which could be assigned to devices

172.16.0.0 255.255.248.0
host address range 172.16.0.1 - 172.16.7.254
172.16.8.0 255.255.248.0
172.16.8.1 - 172.16.15.254
172.16.16.0 255.255.248.0
host address range 172.16.16.1 - 172.16.31.254
172.16.32.0 255.255.248.0
172.16.32.1 - 171.16.63.254
xxxxxxxxx

Sample 1:
DeviceA: 172.16.17.30/20
DeviceB: 172.16.28.15/20
==> to determine if these devices are on the sam subnet or different subnets
Determining the Subnet for DeviceA:
172.16.17.30 10101100.00010000.00010001.00011110
255.255.240.0 11111111.11111111.11110000.00000000
subnet = 10101100.00010000.00010000.00000000
172.16.16.0
Determining the Subnet for DeviceB:
172.16.28.15 10101100.00010000.00011100.00001111
255.255.240.0 11111111.11111111.11110000.00000000
subnet = 10101100.00010000.00010000.00000000
172.16.16.0
VLSM Example
Variable Length Subnet Masks (VLSM)
netA: must support 14 hosts
netB: must support 28 hosts
netC: must support 2 hosts
netD: must support 7 hosts
netE: must support 28 hosts

netA : requires a /28 (255.255.255.240) mask to support 14 hosts
netB : requires a /27 (255.255.255.224) mask to support 28 hosts
netC : requires a /30 (255.255.255.252) mask to support 2 hosts
netD : requires a /28 (255.255.255.240) mask to support 7 hosts
netE : requires a /27 (255.255.255.224) mask to support 28 hosts

The easiest way to assign the subnets is to assign the largest first
netB : 204.15.5.0/27 host address range 1 to 30
netE : 204.15.5.32/27 host address range 33 to 62
netA : 204.15.5.64/28 host address range 65 to 78
netD : 204.15.5.80/28 host address range 81 to 94
netC : 204.15.5.96/30 host address range 97 to 98

CIDR (Classless Interdomain Routing)
to improve both address space utilization and routing scalability in the Internet

[telecom1988]

http://forum.mait.vn/showthread.php?t=7838

[telecom1988]

http://www.ant7.com/forum/forum_post.asp?TID=2493&PN=1

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