1. Internet Networking recitation #1
Subnet + CIDR
Spring Semester 2008, Dept. of Computer Science, Technion

2. Administrative Information
Course site:
webcourse.cs.technion.ac.il/236341
Assistants:
Anna Levin
Roman Sandler
Internet Networking

3. IP Addressing: Original Classful Scheme
IP Address – 32-bit integer globally unique address
Dotted Notation:
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 –
Class B – 14 bits to net id, 16 bits to host id –
Class C – 21 bits to net id, 8 bits to host id –
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 H1 H2 Rest of the Internet H1 H2
Rest of the Internet R .
Network /24
All traffic to /16 H3 H4
A site with two physical networks.
Using subnetting, R advertise these networks as a single network (thus, R accepts all traffic for net )
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 /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).
We need only 6 such sub networks.
The rest 2 sub networks we will partition by subnet mask of /28 bits.
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.
We will get 4 sub networks of 2 hosts in each.
Subnet mask #1 = /27
Subnet mask #2 = /28
Subnet mask #3 = /30
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