1 Spring Semester 2007, Dept. of Computer Science, Technion Internet Networking recitation #1 Subnet + CIDR

2 Internet Networking Administrative Information n Course site: webcourse.cs.technion.ac.il/ n Assistant in Charge:  Itai Dabran n Assistant  Anna Levin

3 Internet Networking IP Addressing: Original Classful Scheme n IP Address – 32-bit integer globally unique address n Dotted Notation: n 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.

4 Internet Networking IP Addressing: Original Classful Scheme n Class A – 7 bits to net id, 24 bits to host id – n Class B – 14 bits to net id, 16 bits to host id – n Class C – 21 bits to net id, 8 bits to host id – n Class D – for multicasting n Class E – reserved for future use (used for private addresses) Weakness n Growth of routing tables in routers  Tens of thousands small (class C) networks.  Each network must be advertised. n Inflexible  Lack of a network classes for mid-sized organization (between class B and C).  Address space will be eventually exhausted

5 Internet Networking Subnet Addressing n 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). n From outside it looks like a single network n Only local routers know about multiple physical networks inside and how to route traffic among them n Host ID is divided into a subnet ID and host ID n Accepted as a standard at 1985 (RFC 950).

6 Internet Networking Subnet Routing n 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. n If the packet is destined to other network then the router sends it to another router. n Otherwise the router sends the packet to the appropriate host on its attached networks.

7 Internet Networking Subnetting - Example R. Network /24 H1H1 H2H2 Network /24 H3H3 H4H Rest of the Internet All traffic to /16 n A site with two physical networks. n Using subnetting, R advertise these networks as a single network (thus, R accepts all traffic for net ) n Internal routing is done according to subnet id (i.e. the third octet of the address).

8 Internet Networking Variable-Length Subnetting n Motivation: Consider the case when an organization has a few network of different sizes. n When we choose the subnet partitioning, we actually define constant number of possible physical subnetworks with maximum number of hosts on them. n 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. n Solution: Variable-Length Subnetting. A subnet partition is selected on a per-network basis.

9 Internet Networking Example – Configuring a Network with Variable-Length Subnetting n We have a network with IP /24 n We need to support next sub networks:  6 networks with 26 hosts  3 networks with 10 hosts  4 networks with 2 hosts n 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).  n We need only 6 such sub networks. n The rest 2 sub networks we will partition by subnet mask of /28 bits.  n We will get 4 sub networks of 14 hosts in each  We need only 3 such sub networks.

10 Internet Networking Example – Configuring a Network with Variable-Length Subnetting n The rest we will partition by subnet mask of /30 bits.  n We will get 4 sub networks of 2 hosts in each. n Subnet mask #1 = /27  n Subnet mask #2 = /28  n Subnet mask #3 = /30 

11 Internet Networking Classless Inter-Domain Routing (CIDR) - RFC 1519 n Routing destinations are represented by network and mask pairs.  Enabling network aggregation; thereby reducing the size of routing table. Examples: n Class A networks are followed by a /8 n Class C networks are followed by a /24 n 8 Class C hosts network is followed by /21 n Such a network has 21 bits of Net-ID, 11 Bits of Host-ID n Contains 2^ 21 Net IDs, and 2^ = Hosts in Each network.