Outline

Lecture 10. Subnetting

Variable Length Subnet Mask (VLSM)
Subnetting & Supernetting Supernetting
Classless Inter-Domain Routing (CIDR)

Giuseppe Bianchi Giuseppe Bianchi

medium org: N x class C? Class B?

Need for subnetting
130.11.0.7 213.2.96.0

R2 Net
130.11.0.0 R3 Net_id-Host_id:
213.2.97.0 place host_id on physical network net_id
R2 Routing Table 131.175.0.2 131.175.0.3 131.175.45.54 131.175.255.254
213.2.98.0
dest Next Hop
130.11.0.0/16 Direct fwd
… … 213.2.99.0
213.2.96.0/24 130.11.0.7
131.175.0.1
213.2.97.0/24 130.11.0.7 Corporate
213.2.98.0/24 130.11.0.7 65534 hosts on a same physical network????
CLASS B:
- performance?
213.2.99.0/24 130.11.0.7 From: 131.175.0.1
- management?
To: 131.175.255.254

Giuseppe Bianchi Giuseppe Bianchi

Idea: further hierarchy level Subnet creation and management

subdivide a network in several subnetworks Give me a class B, please
Private
each subnet = a physical network (Ethernet, FDDI, X.25, 131.175.0.0 for you! Network
ATM, Frame Relay, etc….) InterNIC Administrator

131.175.21.42 131.175.12.33
131.175.12.12 131.175.12.34 131.175.12.0
131.175.21.4 131.175.12.0
131.175.12.254 131.175.21.0
131.175.21.0
Router
Internet 131.175.15.0
Ethernet FDDI 131.175.15.0
131.175.33.0
131.175.x.0
131.175.x.0
Host 131.175.0.0

131.175.12.0
! " " # $ #%
131.175.21.0 131.175.21.1
&
Sub-Net ATM !
Class B network: 131.175.0.0 ' ( #
May use third byte to identify subnet: 131.175.X.0 (or may not!) ) ! "
Giuseppe Bianchi Giuseppe Bianchi

1
 Subnetting Subnet Address & Mask
Class B address example " !#
$%& &%& ' & & &
network prefix ( ) )
(network address)
* $ $ &* $$ & & & & &
+ , )
1 0 NET ID (14bit) HOST ID (16 bit) * "+* !
, ! "- !
- $ .
Extended network prefix "./ 0
(subnet address) & & &
/prefix-length notation
". & & 1&2
(dot decimal notation)
1 0 NET ID (14bit) SUBNET ID (n bit) HOST ID (16-n bit) 0 3&022&2&2 . 4 & & &
0 3&022&1&2 . " 4 5 4
, ( ! 0 3&022&1&2/ 0
Giuseppe Bianchi Giuseppe Bianchi

Remember: subnetting is arbitrary!

Typical class B subnetting Example: subnetting Class C 193.1.1.0 Address

Base net 11000001.00000001.00000001.00000000 193.1.1.0/24

Class B address = /16 network prefix
network address = 131.175.0.0 Class C
1 1 0 NET ID (21bit) HOST ID (8 bit)
/24 prefix
natural mask = 255.255.0.0 Subnetted
Subnet Host id
Subnetted with /24 network prefix 255.255.255.224 1 1 0 NET ID (21bit)
(3 bit) (5bit)
/27prefix
1 0 NET ID (14bit) SUBNET ID (8 bit) HOST ID (8 bit)
Subnet # 0 11000001.00000001.00000001.00000000 193.1.1.0/27
Subnet # 1 11000001.00000001.00000001.00100000 193.1.1.32/27
255.255.255.0 subnet mask Subnet # 2 11000001.00000001.00000001.01000000 193.1.1.64/27
Subnet # 3 11000001.00000001.00000001.01100000 193.1.1.96/27
subnet ID = third number in dotted notation Subnet # 4 11000001.00000001.00000001.10000000 193.1.1.128/27
Subnet # 5 11000001.00000001.00000001.10100000
131.175.21.0 193.1.1.160/27
Subnet # 6 11000001.00000001.00000001.11000000 193.1.1.192/27
No technical reasons to use /24 subnets, but convenient for humans Subnet # 7 11000001.00000001.00000001.11100000 193.1.1.224/27
(subnet boundary clearly visible in dotted notation) Remember: maximum 30(2 -2) hosts attachable to each subnet
5

Giuseppe Bianchi Giuseppe Bianchi

Example: route 193.205.102.36

193 205 102 36
Possible netmask values 1 1 0 0 0 0 0 1 1 1 0 0 1 1 0 1 0 1 1 0 0 1 1 0 0 0 1 0 0 1 0 0

Class C address;
128 64 32 16 8 4 2 1 Outside private domain routed with mask 255.255.255.0
1 0 0 0 0 0 0 0 = 128 network host

1 1 0 0 0 0 0 0 = 192 193 205 102 36

1 1 0 0 0 0 0 1 1 1 0 0 1 1 0 1 0 1 1 0 0 1 1 0 0 0 1 0 0 1 0 0
1 1 1 0 0 0 0 0 = 224
Inside private domain, administrator has set netmask 255.255.255.248
1 1 1 1 0 0 0 0 = 240
255 255 255 248
1 1 1 1 1 0 0 0 = 248 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0
1 1 1 1 1 1 0 0 = 252 Hence, route to subnet address and then to host id, computed as:
1 1 1 1 1 1 1 0 = 254 network subnet host
1 1 1 1 1 1 1 1 = 255 1 1 0 0 0 0 0 1 1 1 0 0 1 1 0 1 0 1 1 0 0 1 1 0 0 0 1 0 0 1 0 0
193.205.102.32 /29 4

Giuseppe Bianchi Giuseppe Bianchi

2
 Subnet routing – 2nd example Router configuration
Core routers unaware of subnetting – route via class mask
… … Classful routing: Net = 162.12.0.0; subnet mask 255.255.255.224
… … … …
162.12.0.0 193.1.1.36 162.12.0.0 145.54.3.5 162.12.0.0 145.54.3.5 All necessary information To other nets
… …
… … … … included in Ipaddr
162.12.2.33 Subnet routing
162.12.34.75 145.54.55.1 162.12.1.11
193.1.1.36 Specific subnet mask 162.12.1.0
To other
… … subnets
162.12.34.64 162.12.1.1
(set by admin) required 162.12.1.12
145.54.0.0
145.54.0.0 145.54.3.5 162.12.1.1
193.1.1.0
193.1.1.0 162.12.2.32 162.12.1.33 Routing Table
default 162.12.9.65 Subnet mask:
255.255.255.224 162.12.34.65
162.12.70.96
193.1.1.1 162.12.70.96
162.12.1.33 162.12.9.65 dest Next Hop
162.12.1.1
162.12.1.0 Direct fwd
162.12.35.128
… …
162.12.2.33 162.12.34.64 Direct fwd 162.12.34.66 162.12.35.128
162.12.34.75
162.12.35.128 162.12.34.66 162.12.34.64
162.12.0.0 193.1.1.36
… … 162.12.70.96 162.12.1.12
routing tables in the Net = 162.12.0.0
131.175.0.0 162.12.34.66 May be quite a complex
Internet: subnet mask = 255.255.255.224 To
131.176.0.0 162.12.34.66 Routing table…
route according to net_id Corporate routers & hosts: 131.175.0.0
default 162.12.1.11 VLSM will help (later)
Use natural class mask Route according to subnet_id 131.176.0.0
Need to KNOW subnet mask
Giuseppe Bianchi Giuseppe Bianchi

Subnetting Example (problem) Subnetting Example (solution?)

algebra algebra
12 hosts 193.1.1.32/27
up to 30 hosts Computation
A (33-62) A 193.1.1.64/27
Computation
Math dept 28 host Math dept up to 30 hosts
22 hosts 193.1.1.96/27 (65-94)
Link-1 up to 30 hosts Link-1
B (97-126) B physics
physics 193.1.1.160/27
10 host up to 30 hosts
(161-190)

Link-2 Link-2

C C

Where are the errors?

193.1.1.0 network 193.1.1.0 network
Giuseppe Bianchi Giuseppe Bianchi

Subnetting Example (solution!)

algebra
193.1.1.32/27
up to 29 hosts Computation
(33-62) A 193.1.1.64/27
Math dept
193.1.1.96/27 Link-1
up to 29 hosts
(65-94)
VLSM
up to 29 hosts
(97-126) B
193.1.1.128/27
physics Variable Length Subnet Mask
193.1.1.160/27
Subnet mask:
/27 255.255.255.224
up to 28 hosts RFC 1009 (1987)
(161-190)
SUBNETS:
Math 193.1.1.96/27 Link-2
193.1.1.192/27
Algebra 193.1.1.32/27
Physics 193.1.1.160/27
Comput 193.1.1.64/27 C
Link-1 193.1.1.128/27
Link-2 193.1.1.192/27 193.1.1.0 network
--- 193.1.1.0/27
--- 193.1.1.224/27
Giuseppe Bianchi Giuseppe Bianchi

3
 A typical problem
Variable Length Subnet Mask
allows more than one subnet mask in the
A x-net-1
same network pc-net
20 host
100 host
A) more efficient use of organization’s IP address space
Link-1
Subnets may significantly vary in relative size (computer B
room = 200 hosts, secretary = 4 hosts…) Link-3
consider a 4 host network with mask 255.255.255.0: wastes
250 IP addresses!
Link-2
B) allows route aggregation, thus reducing routing ws-net
20 host
x-net-2
10 host
information needed
C
Needs further support by routing protocol
100+20+20+10 = 150 total hosts: 1 class C enough (including growth projections).
e.g. RIP1 doesn’t support VLSM
7 subnets (4 LANS + 3 point to point links): 3 bit subnet ID (= up to 8 subnets)
BUT then max 30 host per subnet: no way to accommodate pc-net!!
Giuseppe Bianchi Giuseppe Bianchi

Solution without VLSM 192.168.1.0/25 Using VLSM

(up to 126 hosts)
need 2 class C address! (pc-net) /
192.168.1.0/24 6 7 ( "
(up to 254 hosts) 78 ) 9
pc-net A x-net-1 ( -
192.168.1.0/25 192.168.2.0/27 192.168.1.128/25 7 -
(0-127, 126 host) (0-31, 30 host) (up to 126 hosts) ( :
192.168.2.64/27 192.168.1.128/27
Link-1 (up to 30 hosts) (ws-net)
B
Link-3 192.168.1.160/27 192.168.1.192/28
192.168.2.128/27 (up to 30 hosts) (x1-net) (up to 14 hosts) (x2-net)
192.168.1.192/27
ws-net Link-2 x-net-2
(up to 30 hosts) 192.168.1.208/28
192.168.1.128/25 192.168.2.96/27 (up to 14 hosts)
192.168.2.32/27 192.168.1.224/27
(128-255, 126 host) (32-63, 30 host) (up to 30 hosts) (available)
192.168.1.208/30 (ptp)
C
192.168.1.212/30 (ptp)
192.168.1.0 192.168.2.0 192.168.1.216/30 (ptp)
mask 255.255.255.128 mask 255.255.255.224
192.168.1.220/30 (avail)
Giuseppe Bianchi Giuseppe Bianchi

Final solution with VLSM

1 C address is enough
address pie for our sol.
Available for
further subnets
pc-net A x-net-1
192.168.1.0/25 192.168.1.160/27 Link3 216-219
(0-127, 126 host) (160-191, 30 host)
192.168.1.208/30 Link2 212-215
255.255.255.128 255.255.255.224
Link-1 Link1 208-211
B
Point2point links: Link-3
x-net-2
255.255.255.252 192.168.1.216/30 192-207
PC-net
ws-net Link-2 x-net-2 0-127
192.168.1.128/27 192.168.1.212/30 x-net-1
192.168.1.192/28 160-191
(128-159, 30 host) (192-207, 14 host)
255.255.255.224 255.255.255.240
C WS-net
128-159
192.168.1.0

Giuseppe Bianchi Giuseppe Bianchi

4
 Requirements for VLSM support (1) Routing tables for previous example
' !
" " … … …
! net mask route pc-net
192.168.1.0/25 A x-net-1
' ! … … … (0-127, 126 host) 192.168.1.160/27
" 255.255.255.128 (160-191, 30 host)
192.168.1.208/30
( Link-1 192.168.1.217 255.255.255.224
B
Point2point links: Link-3
255.255.255.252 192.168.1.216/30
New route advertise + mask (or prefix len): 192.168.1.213
131.175.192.0 10000011.10101111.11000000.00000000 ws-net Link-2 x-net-2
255.255.240.0 11111111.11111111.11110000.00000000 192.168.1.128/27 192.168.1.212/30 192.168.1.192/28
prefix /20 (128-159, 30 host) (192-207, 14 host)
255.255.255.224 Router C table 255.255.255.240
Without this feature: manually compiled tables (!!! Human error!!!) C
192.168.1.128 /27 192.168.1.213
192.168.1.0 /25 192.168.1.213
VLSM bottomline: need to use more complex routing protocols 192.168.1.208 /30 192.168.1.213
(e.g. OSPF) even for small org 192.168.1.192 /28 Direct fwd 192.168.1.0 network
192.168.1.192 /28 Direct fwd
Giuseppe Bianchi 192.168.1.212
Giuseppe Bianchi /30 Direct fwd
192.168.1.216 /30 Direct fwd

VLSM engineering VLSM engineering

0 1+ , 0 1+ ,

; , ! 7 ! 7 ; , ! 7 ! 7
<< <<

- -

! !
Substantial reduction of routing table sizes Substantial reduction of routing table sizes
Multiple route aggregation Multiple route aggregation

Giuseppe Bianchi Giuseppe Bianchi

Solution – no route aggregation

Complete example 1 È sufficiente uno /24, es: 64.2.1.0 /24
Acquistando uno spazio di indirizzi il più piccolo possibile, da un provider che network mask dest Una soluzione possibile, con massima aggregazione
gestisce lo spazio 64.2.0.0 /16,
64.2.1.128 /25 64.2.1.129 dei route, è illustrata in figura (si assume che
64.2.1.64 /26 64.2.1.65 il routing esterno alla rete avvenga tramite
-Si divida in sottoreti la rete illustrata in figura in modo da soddisfare alle capacità richieste
64.2.1.48 /28 64.2.1.66 l’interfaccia remota 64.2.100.1)
-Si assegnino indirizzi IP alle interfacce dei router 64.2.1.0 /28 64.2.1.66
-Si mostri la routing table del router R 64.2.1.16 /28 64.2.1.66
still
obscure 0.0.0.0 /0 64.2.100.1 Edificio A
network mask dest
Router R 64.2.1.129 110 hosts
64.2.1.65 64.2.1.128 /25
64.2.100.1 …
Edificio A Edificio D
Router R 110
10 hosts
Edificio B 11 hosts
64.2.1.64 /26 55 hosts 64.2.1.50 64.2.1.17 64.2.1.16 /28
Edificio D
Edificio B 11 hosts
64.2.1.66 Edificio C
110
55 hosts 64.2.1.2
64.2.1.49 10 hosts
Edificio C
10
55 hosts
64.2.1.48 /28 Edificio E
Edificio E
12 hosts 12 hosts

64.2.1.0 /28

Giuseppe Bianchi Giuseppe Bianchi

5
 Solution – final
È sufficiente uno /24, es: 64.2.1.0 /24
Complete example 2
network mask dest Una soluzione possibile, con massima aggregazione Acquistando uno spazio di indirizzi il piu’ piccolo possibile, da un provider che
64.2.1.128 /25 64.2.1.129 dei route, è illustrata in figura (si assume che gestisce lo spazio 64.2.0.0 /16,
64.2.1.64 /26 64.2.1.65 il routing esterno alla rete avvenga tramite -Si subnetti la rete illustrata in figura in modo da soddisfare alle capacità richieste
64.2.1.0 /26 64.2.1.66 l’interfaccia remota 64.2.100.1)
still -Si assegnino indirizzi IP alle interfacce dei router
0.0.0.0 /0 64.2.100.1
obscure -Si mostri la routing table del router R
Edificio A network mask dest
Router R 64.2.1.129 110 hosts
64.2.1.65 64.2.1.128 /25
64.2.100.1 …
Edificio D Edificio A
Edificio B 11 hosts Router R 10 hosts

64.2.1.64 /26 55 hosts 64.2.1.50 64.2.1.17 64.2.1.16 /28 Edificio D

Edificio B 11 hosts
64.2.1.66 Edificio C
64.2.1.2 110 hosts
64.2.1.49 10 hosts
Edificio C
55 hosts
64.2.1.48 /28 Edificio E
Edificio E
12 hosts 12 hosts

64.2.1.0 /28

Giuseppe Bianchi Giuseppe Bianchi

Solution – no route aggregation

È sufficiente uno /24, es: 64.2.1.0 /24 Requirements for VLSM support (2)
network mask dest Una soluzione possibile, con massima aggregazione
64.2.1.128 /25 64.2.1.129 dei route, è illustrata in figura (si assume che
64.2.1.64 /26 64.2.1.200 il routing esterno alla rete avvenga tramite “Longest Match” Forwarding Algorithm
64.2.1.48 /28 64.2.1.49 l’interfaccia remota 64.2.100.1)
64.2.1.0 /28 64.2.1.200 Routing table
64.2.1.16 /28 64.2.1.200
now
clear 0.0.0.0 /0 64.2.100.1 Edificio A 11.0.0.0 /8 Route 1
Router R 64.2.1.49 10 hosts Three
11.1.0.0 /16 Route 2
64.2.1.129 64.2.1.48 /28 IP packet matches
64.2.100.1 …
Edificio D Destination: 11.1.2.5 11.1.2.0 /24 Route 3
Edificio B 11 hosts
64.2.1.128 /25 110 hosts 64.2.1.66 64.2.1.22 64.2.1.16 /28
Longest match = smaller network Best (longest) match
64.2.1.200 Edificio C
64.2.1.77 64.2.1.2
55 hosts

64.2.1.64 /26 Edificio E

12 hosts
no simple aggregation!
64.2.1.0 /28

Giuseppe Bianchi Giuseppe Bianchi

Solution - final
network mask dest
E’ sufficiente uno /24, es: 64.2.1.0 /24
Una soluzione possibile, con massima aggregazione
Example:
Example: VLSM engineering
64.2.1.128 /25 64.2.1.129 dei route, e’ illustrata in figura (si assume che
64.2.1.0 /25 64.2.1.200 Il routing esterno alla rete avvenga tramite
64.2.1.48 /28 64.2.1.49 l’interfaccia remota 64.2.100.1):
0.0.0.0 /0 64.2.100.1 VLSM subnetting of class A 11.0.0.0
Edificio A 11.1.0.0/24 11.1.254.0/28
Router R 64.2.1.49 10 hosts 11.0.0.0/16 11.1.1.0/24 11.1.254.16/28
64.2.1.129 64.2.1.48 /28 11.1.0.0/16 11.1.254.32/28
64.2.100.1 … 11.1.254.0/24
Edificio D 11.2.0.0/16
11 hosts 11.0.0.0/8 11.1.255.0/24 11.1.254.208/28
Edificio B
64.2.1.128 /25 110 hosts 11.253.0.0/16 11.1.254.224/28
64.2.1.66 64.2.1.22 64.2.1.16 /28
11.254.0.0/16 11.254.0.0/19 11.1.254.240/28
64.2.1.200 Edificio C
64.2.1.2 11.255.0.0/16 11.254.32.0/19
64.2.1.77 55 hosts 11.254.64.0/19
64.2.1.64 /26 Edificio E
11.254.192.0/19
12 hosts
11.254.224.0/19
64.2.1.0 /28

Giuseppe Bianchi Giuseppe Bianchi

6
 Route aggregation with VLSM
VLSM allows to hide detailed structure of routing
information for one subnet group from other routers -
reducing routing table Size

11.1.0.0/16
11.0.0.0/16 11.1.0.0/24
Router A 11.1.0.0/16 Router B
11.1.1.0/24
CIDR
11.0.0.0/8
11.253.0.0/16
11.254.0.0/16 11.1.254.0/24 Classless Inter-
Inter-Domain Routing
11.255.0.0/16 11.1.255.0/24
Internet RFC 1517 to 1520 (1993)
11.254.0.0/16 11.1.254.0/24
Router C Router C

11.254.32.0/19 11.1.254.0/28
11.254.64.0/19 11.1.254.16/28
11.1.254.32/28
11.254.192.0/19 11.1.254.224/28
11.254.224.0/19 11.1.254.240/28
Giuseppe Bianchi Giuseppe Bianchi

An historical perspective
N x class C? Class B? The 1992 Internet scenario
130.11.0.7 213.2.96.0 (
Net In early years, Class B addresses given away!
R2 130.11.0.0 R3
Unefficient division into A, B, C classes
213.2.97.0
byte-word: unwise choice (class C too little, class B too big)
The aftermath: much better, e.g. C=10 bits, B=14 bits
R2 Routing Table
213.2.98.0 Projections at the time: class B exhaustion by 1994/95
dest Next Hop
130.11.0.0 Direct fwd
… … 213.2.99.0
213.2.96.0 131.11.0.7
213.2.97.0 131.11.0.7 Corporate
213.2.98.0 131.11.0.7
$
213.2.99.0 131.11.0.7 % <
: ==

Giuseppe Bianchi Giuseppe Bianchi

Routing table growth

The problem 130.11.0.7 213.2.96.0
Corporate has to build 4 R2 Net R3
130.11.0.0
physical networks (e.g. 213.2.96.0/24 213.2.97.0
buildings)
2 )
Example: networks up to 254 hosts R2 Routing Table
213.2.97.0/24 213.2.98.0
Must “buy” 4 IP network dest Next Hop
addresses
130.11.0.0 /xx Direct fwd
213.2.98.0/24 213.2.99.0
… …
213.2.96.0 /24 131.11.0.7
213.2.99.0/24 213.2.97.0 /24 131.11.0.7 Corporate
Why this is bad? 213.2.98.0 /24 131.11.0.7
Corporate 213.2.99.0 /24 131.11.0.7

Giuseppe Bianchi Giuseppe Bianchi

7
 The 1992 Internet scenario Multiple class C assignment
Corporate Network
- 11.0.0.32 20.0.0.6 130.11.0.12 130.11.0.7
213.2.96.8
20.0.0.5 213.2.96.0
Multiple class C allocation dramatic for routing tables
necessary because of Class B exhaustion Net R1 Net R2 Net R3
11.0.0.0 20.0.0.0 130.11.0.0
100.000 entries highly critical for performance 213.2.97.0
» 2M class C: WAY OUT of the capabilities of routing sw & hw
R2 Routing Table 213.2.98.0
Destination Network Next Hop 213.2.99.5
213.2.99.0
20.0.0.0 Direct forward
130.11.0.0 Direct forward
Projections at the time Default routes: suboptimal
End 1990: 2190 routes; end 1992: 8500 routes; 11.0.0.0 20.0.0.5 traffic balancing
End 1995 projection: 70000 routes (critical);
213.2.96.0 130.11.0.7 Core routers: cannot have
End 1995 factual: 30000 routes thanks to default routes (large tables)
classless routing 213.2.97.0 130.11.0.7
Mid 1999: 50000 routes
HW and SW limits on routing
213.2.98.0 130.11.0.7 table lookup time
Routing table updates are
213.2.99.0 130.11.0.7 critical (large tables traveling
among routers for updates)
Giuseppe Bianchi Giuseppe Bianchi

Classless Inter-
Inter-Domain Routing CIDR model
CIDR Classless
3 %% 4
Completely eliminates traditional concepts of Class A, B and
' 6 % 0 0> 70 01 70 03 70 2
C addresses
!5 ) +
network prefix based
6
routers do not make any assumption on the basis of the
! three leading bits
they require an explicit network prefix to determine dividing
32 bits: unwise choice
nobody could expect such an Internet growth point between net_id and host_id
and Internet appliances will have a terrific impact clearly, capability of advertise prefix must be supported by
unwise address assignment in early days routing protocol (e.g. BGP4)
class B addresses with less than 100 hosts are common!!
Projections (RFC 1752): address depletion between 2005 In essence: CIDR = VLSM applied to the
and 2001 WHOLE Internet!!
Ultimate solution: IPv6 (128 bits address!)
Giuseppe Bianchi Giuseppe Bianchi

Cidr addresses CIDR = supernetting

10.23.64.0/20 00001010.00010111.01000000.00000000 Organization assigned 2n class C addresses
with contiguous address space
130.5.0.0/20 10000010.00000101.00000000.00000000
addressing: use network bits with host_id meaning
200.7.128.0/20 11001000.00000111.10000000.00000000
the opposite of subnetting!

Regardless the traditional class, all these addresses are similar! Example: 4 class C addresses appear to networks outside as
All address a network composed of as much as 4094 hosts a single network
Interpreting 200.7.128.0/20: a SINGLE NETWORK, contiguous block of 16 class C addr Natural class C mask
200.7.128.0 200.7.132.0 200.7.136.0 200.7.140.0
200.7.129.0 200.7.133.0 200.7.137.0 200.7.141.0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 00 0 0 0 0 0 0 0 0
200.7.130.0 200.7.134.0 200.7.138.0 200.7.142.0
200.7.131.0 200.7.135.0 200.7.139.0 200.7.143.0 (Super) netmask: 255.255.252.0

Giuseppe Bianchi Giuseppe Bianchi

8
 Routing with CIDR
Supernet Address
213.2.96.8
11.0.0.32 20.0.0.5 20.0.0.6 130.11.0.12 130.11.0.7
4 address-contiguous networks: 213.2.96.0
Net Net Net R3
213.2.96.0 11010101.00000010.01100000.00000000 11.0.0.0 R1 20.0.0.0 R2 130.11.0.0
213.2.97.0 11010101.00000010.01100001.00000000 213.2.97.0

213.2.98.0 11010101.00000010.01100010.00000000 R2 Routing Table

213.2.99.0 11010101.00000010.01100011.00000000 Dest.Net Dest.Netmask Next Hop 213.2.99.5 213.2.98.0
20.0.0.0 255.0.0.0 Direct forward
supernet mask:
130.11.0.0 255.255.0.0 Direct forward
213.2.99.0
255.255.252.0 11.0.0.0 255.0.0.0 20.0.0.5
supernet address: 213.2.96.0/22 213.2.96.0 255.255.252.0 130.11.0.7
Corporate Network
11010101 . 00000010 . 011000 00 . 00000000 Corporate Supernet address: 213.2.96.0/22
11010101 . 00000010 . 011000 00 . 00000000

Giuseppe Bianchi Giuseppe Bianchi

Large networks depolyment Requirements for CIDR support

8 ? *8
Organization assigned 2n class C addresses
may arbitrarily deploy subnetworks with more Routing protocol must carry network prefix
information with each route advertising
than 254 hosts!
This was impossible with class C, as natural netmask all routers must implement a consistent
was /24 forwarding algorithm based on the “longest
match”
BUT Software running on all the subnet hosts
need to accept larger masks than natural one for route aggregation to occur, addresses
e.g. setting netmask = 255.255.252.0 for host IP must be assigned to be topologically
significant
address 193.21.34.54 may be forbidden by sw
Giuseppe Bianchi Giuseppe Bianchi

Route aggregation CIDR allocation

control of internet tables growth topological allocation of ex class-
class-C addresses
1 single advertise for 256 /24!! Multi regional 192.0.0.0 - 193.255.255.255
Europe 194.0.0.0 - 195.255.255.255
200.25.0.0/16 Large ISP
The Internet Others 196.0.0.0 - 197.255.255.255
North America 198.0.0.0 - 199.255.255.255
200.25.16.0/20 Central-South America 200.0.0.0 - 201.255.255.255
200.25.16.0/21
200.25.16.0/24 Pacific Rim 202.0.0.0 - 203.255.255.255
200.25.17.0/24 200.25.24.0/22 Others 204.0.0.0 - 205.255.255.255
200.25.18.0/24 200.25.28.0/23 200.25.30.0/23
200.25.19.0/24 200.25.24.0/24 Others 206.0.0.0 - 207.255.255.255
200.25.20.0/24 200.25.25.0/24 IANA reserved 208.0.0.0 - 223.255.255.255
200.25.21.0/24 200.25.26.0/24 200.25.28.0/24 200.25.30.0/24
200.25.22.0/24 200.25.27.0/24 200.25.29.0/24 200.25.31.0/24 All are class C blocks, since class B blocks are no more allocated…
200.25.23.0/24 Recent trends: “attack” unused class A addresses
Company B Company C Company D
Company A (address space 64.0.0.0/2: from 64.0.0.0 to 126.0.0.0)
Giuseppe Bianchi Giuseppe Bianchi

9
 Exception route
Longest match forwarding IPDEST: 195.0.20.2
11000011.00000000.00001100.00000010
??? 194.0.0.0 /7 (254.0.0.0)
IP packet 11000010.00000000. 00000000. 0
NY PARIS
Destination: 203.22.66.5 Routing table
11001011 . 00010110 . 01000010 . 00000101 ROUTER ROUTER
203.0.0.0 /11 Route 1
Three shorter (cheaper) path
203.20.0.0 /14 Route 2 for this organization...
matches European region
203.22.64.0 /20 Route 3 194.0.0.0 - 195.255.255.255

195.0.16.0 /21 Fuerteventura

Best (longest) match router
R1: 11001011 . 00010110 . 01000010 . 00000101 11000011.00000000. 00001000. 0
R2: 11001011 . 00010110 . 01000010 . 00000101
Lanzarote’s software inc
R3: 11001011 . 00010110 . 01000010 . 00000101
195.0.16.0 - 195.0.23.0
Longest match(R3) = smaller network
But why longest match is ever needed???
Giuseppe Bianchi Giuseppe Bianchi

Common exception route case Common exception route case

200.25.0.0/16 ISP (Albacom) 200.25.16.0/21 200.25.0.0/16 ISP (Albacom)

The Internet The Internet

Organization A Organization A

199.32.0.0/16 ISP (Eunet) 199.32.0.0/16 ISP (Eunet)

200.25.16.0/21
200.25.16.0/21
At a point in time, organization A selects Eunet as new ISP!
Then organization A keeps the same address block
Best thing to do (for the Internet): obtain a new block of addresses Eunet is in charge to advertise the new block, too, by
and renumber injecting in the internet more specific route infos
virtually impossible for a reasonably complex organization… This has created a new entry in routing tables, to be solved
and even think to organizations that re-sells subnets...
with longest match
Giuseppe Bianchi Giuseppe Bianchi

The open problems of CIDR

Address blocks for private Internets
& + ( !5
(RFC 1918)
$ % #@ ' $ ! A B
"
% C IANA-
IANA-Allocated,
Allocated, Non-
Non-Internet Routable,
Routable,
*& IP Address Schemes
<
Class Network Address Range
Address ownership (portable blocks): dramatic
» Proposals (not accepted) to allows ownership only up to /9 ISPs A 10.0.0.0-10.255.255.255
» Current “rule”: ownership starts from 8192 host networks (/19) B 172.16.0.0-172.31.255.255
Address lending
» Renumbering necessary when changing ISP C 192.168.0.0-192.168.255.255
4& + !#
D #E ' 6 % 03 0> To be used by private organizations not connected to the Internet
unlikely, as they are viewed as assets!! No need to ask to IANA or InterNIC for these addresses.
Use Network Address Translator when external connectivity needed
Giuseppe Bianchi Giuseppe Bianchi

10
 IPv6
Network Address Translator
(IP next generation - IPng)
IPng)
“Inside” Network “Outside” Network
The ultimate address space solution
NAT 128 bit addresses
10.0.0.2 192.69.1.1
10.0.0.2
Internet
some other very important corrections and
improvements to IPv4
10.0.0.3
although mostly designed to be as close as possible
Source Address
to IPv4
NAT Table
Inside Local Inside Global Prices to pay:
IP Address IP Address

10.0.0.2 192.69.1.1
Double IP header size (40 bytes versus 20)
10.0.0.3 192.69.1.2
Difficult and slow transitory from IPv4 to IPv6
Map external address with Internal ones (may be a subset)
Giuseppe Bianchi Giuseppe Bianchi

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