Subnets

Carrier PE routers have a global routing table which is about 700,000+ and it is always on the rise.

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Provide Edge (PE)

E is the abbreviation of Edge. PE is “border router”. The border router is usually located at the border of the operator and physically connected with others. PE routers and PE routers of other carriers exchange routes through BGP routing protocols.

1.Learning the routing table of other operators (incoming)

2.Advertising your own routing table (outgoing)

Through the above routing exchange, the local PE has a global routing table.

Shouldn't the globally available IP addresses be 2 billion+?

The IP address is a total of 32 digits. In theory, there are 232 = 4.29 billion IP addresses, excluding multicast, private, test, and reserved IP. The available IP addresses are more than 2 billion+.

Is the current memory of the router capable of accommodating 2 billion+ massive routing entries?

On a Cisco router, it takes about 100 bytes to save a BGP route entry. How many bytes do you need to save 2 billion route entries?

2 * 109* 100 = 2*1011 = 200 G bytes

This massive memory demand is difficult to meet even the most high-end routers.

Since the router can't accommodate every IP addresses, can we summarize the IP addresses with a uniform prefix which greatly reduces the entry of the routing table and introduces the magic effect of the network mask again?

What is the shortest network mask length?

0

Where is the mask length "0" used?

For example, 0.0.0.0/0, the previous article has described this magic route in detail.

The computers and smartphones of hundreds of millions of users on the globe only need to have this 0.0.0.0/0 route to access all the IP address resources of the Internet. In short, 0.0.0.0/0 represents all routing entries on the Internet.

What is the longest network mask?

32

Where is the mask length "32" used?

For example, 1.1.1.1/32, it means an IP address of 1.1.1.1.

The user dials the Point-to-Point Protocol over Ethernet (PPPoE), and the PPPoE server assigns the IP address of the 32-bit mask to the PPPoE client, which is assumed to be 1.1.1.1/32.

To sum up, the mask length 0 represents all IP addresses in the world but the mask length 0 has only one use case which is 0.0.0.0/0. Other than that, no other usage, and there is no combination of 1.1.1.1/0!

The mask length 32 represents only one IP address such as 1.1.1.1/32 and 1.1.1.1 is a meaning, representing an individual IP address.

Mask length "31"

The mask length 31 represents that a network segment has 2 IP addresses.

Calculation formula: number of network segment IP addresses = 2 (32-31) = 2

1.1.1.0/31, this route actually represents two IP addresses: 1.1.1.0, 1.1.1.1

What about 1.1.1.2/31? This route represents two IP addresses will be 1.1.1.2, 1.1.1.3 and so on. . .

What we called the 31-bit mask that means the 32-bit IP addresses (from left to right), can be 0, or can be 1. Where the mask is not covered, it can be any combination of 0 and 1.

Since the 31-bit mask only able for 2 IP addresses, it is usually used on a point-to-point link, each IP address on both side and no IP address is wasted!

If a 30-bit mask is used on a point-to-point link then 2 IP addresses will be wasted.

Mask length "30"

According to the calculation formula

Number of IP addresses on the network segment = 2 (32-30) = 4

A route with a mask length of 30 represents 4 IP addresses.

Mask length "29"

Number of network segment IP addresses = 2 (32-29) = 8

A route with a mask length of 29 represents 8 IP addresses.

Mask length “8"

Number of IP addresses on the network segment = 2[(32-8) = 224] = 16777216

A route with a mask length of 8 represents 16777216 IP addresses.

The difference between 1.0.0.0/8 and 1.1.1.1/8

1.0.0.0/8 is a collection of all IP addresses with a prefix of 1. If it is better to write 1.x.x.x/8, x can be any value from 0 to 255. This aggregation has a total number of 16,777,216 IP addresses.

1.0.0.0/8 is a routing entry containing 16777216 IP addresses, usually found in the router's routing table with automatically summarized by the router or by manual.

1.1.1.1/8 is an individual. The IP address of this individual is 1.1.1.1. Obviously, it belongs to the 1.0.0.0/8 large collection because it belongs to the 1.x.x.x collection, isn't it?

1.1.1.1/8 is usually configured on the host interface for its use.

By using a network mask to group IP addresses with the same prefix into a single prefix which helps the global routing table greatly reduced.

As long as the shorter the network mask, the more IP addresses are summarized, can we use a network mask of length 8 to summarize the routes?

For examples

1.0.0.0/8

2.0.0.0/8

3.0.0.0/8

. . .

221.0.0.0/8

222.0.0.0/8

223.0.0.0/8

Then the global routing table except for the private IP (10.x.x.x), reserved IP (169.x.x.x) and native IP (127.x.x.x) then we only need more than 200 routing entries can be expressed, right?

No, because an IP address prefix does not belong to one single provider so it cannot be summarized.

For example, the address prefix 1.x.x.x has not belonged to a single provider. If 1.1.x.x belongs to a US carrier and 1.2.x.x belongs to a European carrier, these IP addresses cannot be summarized.

If they summarized together, the 1.0.0.0/8 route is to the US or Europe?

Simple! It is classified as 1.1.0.0/16 in the United States and 1.2.0.0/16 is a carrier belonging to Europe.

A packet with the destination IP address of 1.1.1.1 matches the route to 1.1.0.0/16 and is sent to the US carrier.

A packet with the destination IP= 1.2.1.1 matches the route to 1.2.0.0/16 and is sent to the European carrier.

In the early days of the Internet, due to the memory limitations of the router and the small network bandwidth, BGP has a restriction on the mask length of the route, which needs ≤19.

If the longest 19-bit mask, how many routes have been limited on the global routing table?

219= 524288

Later, as the performance of the router continued to increase and the memory as well, the IP address became more discontinuous, it was increasingly unrealistic to use a 19-bit network mask. Later, BGP relaxed the limitation on the length of the network mask as long as it is not greater than 21 is fine.

21-bit mask length, how much is the global routing table limited?

221= 2097152