Subnet

1
Subnet Classless Address Extensions

• Linda Wu
• (CMPT 471 2003-3)

2
Content

• Motivation
• Transparent routers
• Proxy ARP
• Subnet addressing
• Classless addressing
• Reference chapter 10

3
Motivation

• Problem network growth will exhaust IPv4 address
  space eventually
• Solution minimize the number of addresses used
• Avoid assigning netid whenever possible
• Share one netid among multiple networks
  transparent router, proxy ARP, subnet addressing
• Arbitrary assignment classless addressing

4
Transparent Routers

• A special router T, called transparent router,
  connects WAN and LAN
• Other hosts and routers on the WAN do not know
  Ts existence
• The LAN does not have its own netid the hosts in
  the LAN are assigned unused addresses in the WAN
• T sends packets from the WAN to the appropriate
  host in the LAN
• T accepts packets from the LAN and routes them
  across WAN to the destination

5
Transparent Routers (cont.)

• Advantages
• One netid is shared by multiple networks fewer
  network addresses are required
• Support load balancing
• Disadvantages
• Only work with WAN that has a large address space
• Do not provide all the same services as
  conventional routers

6
Proxy ARP

• ARP
• Address Resolution Protocol
• Maps IP addresses to physical addresses
• Proxy
• an application that closes a straight path
  between 2 networks and prevents the crackers from
  obtaining internal addresses and details of a
  private network

7
Proxy ARP (cont.)

• How proxy ARP works?
• Main network and hidden network share the same
  netid
• A router, R, connects these 2 networks
• R knows which hosts lie on which physical
  networks, and maintains the illusion that only
  one network exists
• A in the main network sends packets to E in the
  hidden network
• A broadcasts ARP request for Es physical addr.
• R responses ARP request by sending back its own
  physical addr.
• A sends the packet destined for E to R
• R forwards the packet destined for E over the
  hidden network

8
Proxy ARP (cont.)

• Advantages
• One netid is shared by multiple networks
• Proxy ARP can be added to a single router without
  disturbing other hosts or routers on the network
• Disadvantages
• The network must use ARP for address resolution
• Cannot be generalized to more complex network
  topology
• Rely on managers to maintain tables of machines
  and addresses manually

9
Subnet Addressing

• Subnet addressing, subnetting
• A network is divided into several smaller subnets
• Each subnet has its own subnet address
• Subnets appear as a single network to the rest of
  the internet
• The router attached to the subnets knows the
  network is physically divided into subnets

10
Subnet Addressing (cont.)

• Subnetting Example

11
Subnet Addressing (cont.)

• Subnetting address

141 14 191 10
netid hostid
Without subnetting Without subnetting Without subnetting
Hierarchy addressing netid subnetid hostid
141 14 192 192
netid subnetid hostid
With subnetting With subnetting With subnetting With subnetting With subnetting
( 405 ) 864 - 8902
Area code Area code Area code Exchange Connection Connection
Hierarchy in telephone number Hierarchy in telephone number Hierarchy in telephone number Hierarchy in telephone number Hierarchy in telephone number Hierarchy in telephone number
12
Subnet Addressing (cont.)

• Mask
• a 32-bit binary number that gives the network
  address when bitwise ANDed with an IP address
• e.g.
• IP address 123.24.3.1 (class B)
• Mask 11111111 11111111 00000000 00000000
• IP mask 123.24.0.0 (network address)

13
Subnet Addressing (cont.)

• Default masks
• Masks for class A, B, C addresses
• 1s preserve the netid
• 0s set the hostid to 0
• Number of 1s is predetermined 8/16/24

Class Binary mask Dotted-decimal mask
A 11111111 00000000 00000000 00000000 255.0.0.0
B 11111111 11111111 00000000 00000000 255.255.0.0
C 11111111 11111111 11111111 00000000 255.255.255.0
14
Subnet Addressing (cont.)

• Subnet mask
• Change some of the leftmost 0s in the default
  mask to 1s to make a subnet mask
• Preserve netid and subnetid, set hostid to 0
• Contiguous subnet mask (recommended)
• 11111111 11111111 11000000 00000000
• Noncontiguous subnet mask
• 11111111 11111111 00110000 001000000

Default mask 255.255.0.0 Default mask 255.255.0.0
11111111 11111111 00000000 00000000
Subnet mask 255.255.224.0 Subnet mask 255.255.224.0 Subnet mask 255.255.224.0
11111111 11111111 111 00000 00000000
15
Subnet Addressing (cont.)
Subnet mask 255.255.224.0
Bitwise AND
141.14.72.24 IP address
141.14.64.0 Network address
16
Subnet Addressing (cont.)

• Subnet design example
• A company is granted network address 200.16.64.0
  (class C). It needs 6 subnets. Design the subnet.
• of 1s in the default mask 24 (class C)
• 6 subnets lt 23 need 3 more 1s in the subnet mask
• Total of 1s in the subnet mask 24 3 27
• Total of 0s in the subnet mask 8 3 5
  (hostid bits)
• Mask is 11111111 11111111 11111111 11100000, or,
  255.255.255.224
• of hosts per subnet 25 32

17
Subnet Addressing (cont.)

• Subnet address ranges

1st 200.16.64.0 200.16.64.31
2nd 200.16.64.32 200.16.64.63
3rd 200.16.64.64 200.16.64.95
4th 200.16.64.96 200.16.64.127
5th 200.16.64.128 200.16.64.159
6th 200.16.64.160 200.16.64.191
7th 200.16.64.192 200.16.64.223
8th 200.16.64.224 200.16.64.255
18
Subnet Addressing (cont.)

• Fixed-length subnetting
• All 1s or all 0s subnet is not recommended
• All 1s and all 0s host addresses are reserved
• Variable-length subnetting
• No single subnetid partition works for all
  organizations
• An organization may select subnetid partition on
  a per-network basis all hosts and routers
  attached to the network must follow the partition

Subnet bits of subnets Hosts per subnet (class B)
0 1 (20) 65534 (216 2)
2 2 (22-2) 16382 (214 2)
8 254 (28-2) 254 (28-2)
19
Subnet Addressing (cont.)

• Variable-length subnetting example
• A class C site has 5 subnets with host numbers
  60, 60, 60, 30, 30
• 2 bits in subnetid? No, only 4 subnets.
• 3 bits in subnetid? No, at most 32 hosts per
  subnets.

router
20
Subnet Addressing (cont.)

• Subnet broadcasting
• Subnet broadcast address
• hostid is all 1s
• 3-tuple form netid, subnetid, -1, -1 means
  all 1s.
• netid, -1, -1
• Means deliver packet to all hosts with network
  address netid, even if they are in separate
  physical subnets
• Operationally, such broadcasting make sense only
  if routers that interconnect the subnets agree to
  propagate the packets to all subnets

21
Classless Addressing

• Also called supernetting
• Combine several address blocks to create a larger
  address range supernet
• Instead of using a single netid for multiple
  subnets, it allows a networks addresses to span
  multiple netids
• E.g., an organization that needs 1000 addresses
  can be granted 4 class C blocks instead of 1
  class B block
• X.Y.32.0 X.Y.32.255
• X.Y.33.0 X.Y.33.255
• X.Y.34.0 X.Y.34.255
• X.Y.35.0 X.Y.35.255

22
Classless Addressing (cont.)

• Address block assigning
• Choose address blocks randomly
• The routers outside of the supernet treat each
  block separately
• Each router has N entries in its routing table, N
  of blocks therefore, increase the size of
  the routing table tremendously

23
Classless Addressing (cont.)

• Choose address blocks based on a set of rules so
  that each router has only one entry in the
  routing table required by CIDR (Classless
  Inter-Domain Routing)
• of blocks is a power of 2 (1, 2, 4, 8 )
• The size of each block is a power of 2
• The blocks are contiguous in the address space
  (no gaps between the blocks)
• The size of supernet ( of blocks) (size of
  each block) a power of 2
• The first address can be evenly divisible by
  supernet size

24
Classless Addressing (cont.)

• Blocks defining in different addressing schemes
• Block first address, last address
• Classful address
• one block, default mask is always known
• the first address only can define the block
• Subnetting
• the first address in the subblock (subnet) and
  subnet mask define the subblock
• Supernetting
• the first address of the supernet and supernet
  mask define the superblock
• IP address supernet mask first address
  (network address)

25
Classless Addressing (cont.)

• Supernet mask
• The reverse of a subnet mask
• Has less 1s than the default mask for this class

Subnet mask Divide 1 network into 8 subnets Subnet mask Divide 1 network into 8 subnets
11111111 11111111 11111111 111 00000
Subnetting
Default mask (class C) Default mask (class C)
11111111 11111111 11111111 000 00000
Supernetting
Supernet mask Combine 8 networks into 1 supernet Supernet mask Combine 8 networks into 1 supernet
11111111 11111111 11111 000 000 00000
26
Classless Addressing (cont.)

• Supernet mask examples
• A supernet is made out of 16 class C blocks, what
  is its supernet mask?
• Block 16 24
• Change the last 4 1s in the default mask (class
  C) to 0s to get the supernet mask
• 11111111 11111111 11111111 00000000
• 11111111 11111111 11110000 00000000

27
Classless Addressing (cont.)

• A supernet with mask 255.255.248.0 includes an
  address 205.16.37.44, what is the address range?
• First address
• 205.16.37.44 AND 255.255.248.0
• 205.16.32.0
• (11001101 00010000 00100000 00000000)
• Mask 11111111 11111111 11111000 00000000, 1s 21,
  0s 11
• Last address 205.16.39.255 (11001101 00010000
  00100111 11111111)

28
Classless Addressing (cont.)

• Slash notation (CIDR notation)
• A.B.C.D/n
• For identifying a CIDR block
• A.B.C.D an IP address
• n of bits that are shared in every address in
  the block, i.e., of 1s in the mask
• Prefix common part of the address range (similar
  to netid), prefix length n
• Suffix varying part of the address range
  (similar to hostid), suffix length 32 - n

29
Classless Addressing (cont.)

• Relationship between mask and prefix length

/n Mask /n Mask
/1 128.0.0.0 /12 255.240.0.0
/2 192.0.0.0 /16 255.255.0.0 (class B)
/3 224.0.0.0 /24 255.255.255.0 (class C)
/8 255.0.0.0 (class A) /32 255.255.255.255
Class A a.b.c.d/8 Class B a.b.c.d/16 Class C
a.b.c.d/24
30
Classless Addressing (cont.)

• Subnetting with classless addressing
• Increase supernet prefix length (n) to define the
  subnet prefix length
• Example an organization is granted the block
  130.34.12.64/26. It needs to have 4 subnets. What
  is the subnet address and address range for each
  subnet?
• Prefix length 26, suffix length 6 ? of
  addresses in the block 26 64
• 4 subnets ? 16 addresses per subnet
• 4 subnets ? subnet prefix /28 (2 more 1s in the
  mask)

31
Classless Addressing (cont.)

• Subnet address ranges
• 1st 130.34.12.64/28 130.34.12.79/28
• 2nd 130.34.12.80/28 130.34.12.95/28
• 3rd 130.34.12.96/28 130.34.12.111/28
• 4th 130.34.12.112/28 130.34.12.127/28

32
Classless Addressing (cont.)

• Reserved CIDR blocks
• Private addresses, unroutable addresses
• Used with private networks
• Never assigned to networks in the global Internet
• Router in the global Internet knows they are
  reserved addresses, and can detect it if a packet
  destined to the reserved address accidentally
  reaches the Internet

33
Classless Addressing (cont.)

• Reserved CIDR blocks list

Prefix First address Last address
10/8 10.0.0.0 10.255.255.255
172.16/12 172.16.0.0 172.31.255.255
192.168/16 192.168.0.0 192.168.255.255
169.254/16 169.254.0.0 169.254.255.255