Subnet + CIDR. Internet Networking recitation #1. 11: , Internet Networking,

Transcription

1 Subnet + CIDR Internet Networking recitation #1 11: , Internet Networking,

2 Administrative Information Course site: webcourse.cs.technion.ac.il/ Grading policy: 15% homeworks + 85% final exam 6 home assignments. Submission is in pairs. Teaching Assistants: Yuval Nezri Office location: Taub 312 Teaching hours: Monday 14:30-15:20, Ulmann 303 Tuesday 9:30-10:20, Taub 3 Reception hour: Tuesday 11:30 12:30 yuvalnezri@gmail.com 11: , Internet Networking,

3 IP Addressing IP Address 32-bit integer, globally unique address. 11: , Internet Networking,

4 Dotted Notation 11: , Internet Networking,

5 Original Classful Scheme 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. 11: , Internet Networking,

6 Original Classful Scheme Class A Class B Class C Why is this method not scalable? 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 11: , Internet Networking,

7 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 considerations. Administration (e.g. different 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. IP Address is divided into a subnet ID and a host ID. Accepted as a standard at 1985 (RFC 950). 11: , Internet Networking,

8 Subnet Routing When a router gets a packet, it isolates by using Net mask the packet net address. Each routing entry contains a net mask. Routing is done on a longest-match basis. If the packet is destined to another network then the router forwards it to another router. Otherwise the router sends the packet to the appropriate host on its attached networks. 11: , Internet Networking,

9 Classless Inter-Domain Routing Routing destinations are represented by network and mask pairs. Enabling network aggregation; thereby reducing the size of routing tables. 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. 11: , Internet Networking,

10 Subnetting - Example Network / Rest of the Internet All traffic to /16 R H 1 H 2. Network / H 3 H 4 A site with two physical networks. Using subnetting, R advertises these networks as a single network (thus, R accepts all traffic for network ) Internal routing is done according to subnet id (i.e. the third octet of the address). 11: , Internet Networking,

11 Variable-Length Subnetting Motivation: Consider the case when an organization has a few networks of different sizes. When we choose the subnet partitioning, we actually define a constant number of possible physical subnetworks with maximum number of hosts in 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. 11: , Internet Networking,

12 Variable-Length Subnetting 11: , Internet Networking,

13 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 only need 3 such sub networks. 11: , Internet Networking,

14 Example Configuring a Network with Variable-Length Subnetting The rest we will partitioned by subnet mask of /30 bits We will get 4 sub networks of 2 hosts each. Subnet mask #1 = / Subnet mask #2 = / Subnet mask #3 = / : , Internet Networking,

15 Example Configuring a Network with Variable-Length Subnetting Network address Subnet size 2^5-2=30 hosts 2^5-2=30 hosts 2^5-2=30 hosts 2^5-2=30 hosts 2^5-2=30 hosts 2^5-2=30 hosts 2^4-2=14 hosts 2^4-2=14 hosts 2^4-2=14 hosts 2^2-2=2 hosts 2^2-2=2 hosts 2^2-2=2 hosts 2^2-2=2 hosts 11: , Internet Networking,

16 Questions? 11: , Internet Networking,