In a perfect would, we might refer to each grouping as a subnet, based on their network subnet mask, from which their interconnectivity is defined at the base of the network architecture [+1]. In this way all nodes would be relatable to their grouping based on their use of distinct private IP addresses under the appropriate geographically stratified subnets.
But this term is meaningless foreign jargon in our societies [-1].
The terms "silo" and "information silo" carry perjorative connotations, and that negative portrayal may be inappropriate for briefing less-technical audiences, especially senior management [-1].
Despite this, this term is especially well-suited to explaining the infinitely complex layers of a network architecture, because it offers an easy visual reference to farmland silos that stand tall, stiff and unopposed. This term therefore also subtly connotes a level of isolation or separation from other network elements, which in this case is quite accurate [+1].
While the term "environment" is frequently used to describe network architectures, I believe the usage more often indicates the entirety of an organization's interconnected networks, i.e. a secure, closed, private network would be considered a separate environment by virtue of it being closed from the other network [-1].
To add further complication, the term "environment" is the de facto go-to term for developers and programmers when describing programming environments often facilitated by virtual machines; the term is often cherrypicked into the frequent usage "dev[eloper] environment" [-1].
Nevertheless this term does accurately convey the intended meaning without inducing marked cognitive dissonance in our societies, especially in the usage "network environment" [+1].
Others, like "Cluster", "Array" and "Cloud"
The term "cluster" is notably disputed in the answers on this page for their technical accuracy in signifying the intended meanings, likely because of the prevalence of varied usages [-1].
This puts "cluster" in a similar position to "array", which since 1987 adopted the usage Redundant Array of Inexpensive Disks (RAID).
In brief, terms that are in frequent, strata-independent usage for specific purposes in other information technology domains, may pose a greater challenge to repurpose for this particular meaning.
An alternative term that could suit this purpose is "layer", especially in the usage "network layer", as it following the same network analogy as The Onion Router, known widely today as the anonymity network Tor [+1].
"Layer" could be seen to be particularly well-suited to this purpose because of it's wide acceptance relating to systems isolation [+1].
In order to incorporate the geographical distinction, some "layers" could be referred to as "geo[graphical] layers", while others might be "global layers" because they are not specifically stratified geographically.
However, it is arguable that "layer" connotes a higher degree of network isolation than the degree indicated in your question. For that reason, "layer" still may not be the best suited term for this purpose [-1].
The Real Question
Language is very fluid, dynamic and in a constant state of flux. English is a key example of that constant change, being heavily impacted by contact and coexistance with other languages. It's 2014. This industry is dominated by brands with names linking together phonemes that do not always comprise otherwise intelligible morphemes or that are unintelligible in their combination: twitter, Google, tumblr, instagram, feedly.
But that's just brands, the jargon comprising our IT geek speak could be seen to be even more esoterically derived, especially in common usages like "ethernet", which might otherwise be understood as a some sort of fishing net-like tool in an existential alternate universe.
Given the malleablity of language, there is little preventing you from innovating a semantically distinct, new term specifically for this purpose and then advocating it's usage. Consider the brands Saltstack, Ansible, Chef and Puppet: two seem centred in the kitchen and food, and the other two are centered in science fiction literature and 20th century geek speak.
You may be best off using innovative terms from the same origins. Consider: each node (end-user computer or server) a "chocolate cookie", all on various "cookie trays" (Organizational Units (OUs) in an Active Directory), with various types of "chocolate chips" applied to each "chocolate cookie" as they "bake" (push/pull/poll a server for Group Policy Objects) in various "ovens" (geographical groupings) some of which are processed in the same "master pastry factories" (datacentres), or "supplier warehouses" (outsourced cloud Infrastructure-as-a-Service).
This terminology may sound outlandish, but it's not that far off from the words of the Circus used by John le Carré in Tinker Tailor Solider Spy.
So the real question is: How innovative are you? versus How innovative are you allowed to be? versus How innovative ought you to be?