Download Recitation1-Subnets

Internet Networking
recitation #1
Subnet + CIDR
Spring Semester 2007, Dept. of Computer Science, Technion
1
2
Administrative Information

Course site:
webcourse.cs.technion.ac.il/236341

Assistant in Charge:

Itai Dabran
Assistant


Anna Levin
Internet Networking
3
IP Addressing:
Original Classful Scheme

IP Address – 32-bit integer globally unique address

Dotted Notation: 132.68.37.54

IP Classes – dividing an address to net id and host id


The prefix (net id) identifies a network.
The suffix (host id) identifies a host on this network.
Internet Networking
4
IP Addressing:
Original Classful Scheme





Class A – 7 bits to net id, 24 bits to host id 1.0.0.0 – 126.0.0.0
Class B – 14 bits to net id, 16 bits to host id 128.0.0.0 – 191.255.0.0
Class C – 21 bits to net id, 8 bits to host id 192.0.0.0 – 223.255.255.0
Class D – for multicasting
Class E – reserved for future use (used for private addresses)
Weakness


Growth of routing tables in routers
 Tens of thousands small (class C) networks.
 Each network must be advertised.
Inflexible
 Lack of a network classes for mid-sized organization (between class
B and C).
 Address space will be eventually exhausted
Internet Networking
5
Subnet Addressing

A site has a single IP network address assigned to it, but has two or
more physical networks.
 Different technologies.
 Limits of technologies.
 Network congestion.
 Security consideration.
• VLAN – separate one physical network into a few logical
networks.
 Administration (e.g. deferent departments in academic institute).

From outside it looks like a single network
Only local routers know about multiple physical networks inside and
how to route traffic among them
Host ID is divided into a subnet ID and host ID
Accepted as a standard at 1985 (RFC 950).



Internet Networking
6
Subnet Routing

When a router gets a packet, it isolates by Net mask the
packet net id address.
Each routing entry contain a net mask.
 Routing is done on a longest-match basis.
If the packet is destined to other network then the router
sends it to another router.



Otherwise the router sends the packet to the appropriate host
on its attached networks.
Internet Networking
7
Subnetting - Example
Network 128.10.1.0/24
128.10.1.1
Rest of the
Internet
H1
128.10.1.2
H2
R
.
Network 128.10.2.0/24
All traffic to
128.10.0.0/16



H3
128.10.2.1
H4
128.10.2.2
A site with two physical networks.
Using subnetting, R advertise these networks as a single network (thus,
R accepts all traffic for net 128.10.0.0)
Internal routing is done according to subnet id (i.e. the third octet of the
address).
Internet Networking
8
Variable-Length Subnetting




Motivation: Consider the case when an organization has a
few network of different sizes.
When we choose the subnet partitioning, we actually define
constant number of possible physical subnetworks with
maximum number of hosts on them.
Difficult to keep small (waist of subnet numbers) and big (the
host id needs more bits) sub networks and there could be
unnecessary spending of address space.
Solution: Variable-Length Subnetting. A subnet partition is
selected on a per-network basis.
Internet Networking
9
Example – Configuring a Network
with Variable-Length Subnetting

We have a network with IP 202.128.236.0/24

We need to support next sub networks:




6 networks with 26 hosts
3 networks with 10 hosts
4 networks with 2 hosts
If we take subnet mask of /27 bits then we can get 8 sub networks of
30 hosts (all 0’s and all 1’s of host addresses are reserved).

11111111.11111111.11111111.11100000
We need only 6 such sub networks.

The rest 2 sub networks we will partition by subnet mask of /28 bits.

11111111.11111111.11111111.11110000
We will get 4 sub networks of 14 hosts in each



We need only 3 such sub networks.
Internet Networking
10
Example – Configuring a Network
with Variable-Length Subnetting

The rest we will partition by subnet mask of /30 bits.

11111111.11111111.11111111.11111100

We will get 4 sub networks of 2 hosts in each.

Subnet mask #1 = /27


Subnet mask #2 = /28


11111111.11111111.11111111.11100000
11111111.11111111.11111111.11110000
Subnet mask #3 = /30

11111111.11111111.11111111.11111100
Internet Networking
11
Classless Inter-Domain Routing (CIDR) - RFC 1519

Routing destinations are represented by network and mask
pairs.
 Enabling network aggregation; thereby reducing the size
of routing table.
Examples:

Class A networks are followed by a /8

Class C networks are followed by a /24

8 Class C hosts network is followed by /21

Such a network has 21 bits of Net-ID, 11 Bits of Host-ID

Contains 2^21 Net IDs, and 2^11-2 = Hosts in Each network.
Internet Networking