diff --git a/.zk/notebook.db b/.zk/notebook.db
index ce34cda..0d3495a 100644
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diff --git a/zk/Internet_fundamentals.md b/zk/Internet_fundamentals.md
index cb44a5c..f18d14a 100644
--- a/zk/Internet_fundamentals.md
+++ b/zk/Internet_fundamentals.md
@@ -7,4 +7,24 @@ created: Saturday, August 03, 2024
 # Internet fundamentals
 
 The internet is a _[network](Network_fundamentals.md) of networks_. A globally
-connected set of computer networks that all use a suite of common protocols.
+connected set of computer networks that all use a suite of common
+[protocols](Network_protocols.md).
+
+The protocols governing the communication between networks on the internet are
+specified in the **Internet Protocol Suite**.
+
+This combines two protocols:
+
+- **TCP**
+  - The Transmission Control Protocol
+- **IP**
+  - The Internet Protocol
+
+Hence the Internet Protocol Suite is also known as "TCP/IP".
+
+The TCP/IP is a network stack comprising four layers. From the top:
+
+![TCP/IP stack diagram](../img/TCP_IP.png)
+
+The Link Layer ... The Internet Layer ... The Transport Layer ... The
+Application Layer ...
diff --git a/zk/Network_protocols.md b/zk/Network_protocols.md
index 70ea9e2..d707bdf 100644
--- a/zk/Network_protocols.md
+++ b/zk/Network_protocols.md
@@ -9,4 +9,12 @@ created: Saturday, August 03, 2024
 Protocols are agreed standards specifying and governing communication between
 nodes on a [network](Network_fundamentals.md).
 
-Network protocols are typically _layered_.
+Network protocols are typically _layered_. The implmentation of a network layer
+is a _stack_.
+
+Stacks enshrine encapsulation: not every layer has to be directly concerned with
+every other layer. Only adjacent layers that interface with each other need to
+be in contact.
+
+A prime example of a layered network stack is the
+[internet_protocol_suite](Internet_fundamentals.md).